How to hide an urban landmark?

Picture an alternative history Second World War Paris.

The Third Reich must destroy the Notre-Dame de Paris, and the inhabitants must prevent this.

The French do not have access to anti-air weaponry (non-weaponry defences are permitted), the Third Reich has bombers.

Due to sabotage efforts and the primitive nature of maps in this alternate world, the Third Reich has no useful maps of the city. Instead, the pilots have been trained to recognise the cathedral based on tourist photography.

Can the citizenry, together with help from the Allies, prevent the bombers from spotting the cathedral?

A week may pass before the first bombers arrive, and anti-air weaponry will be available within two weeks of the first arrivals.

• Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Jun 6 at 12:45

During WWII the city of Alexandria, in Egypt, was under menace of being bombed by the German bombers.

Since it was known that the bombing would have taken place overnight, it was decided to build in the desert an exact replica of Alexandria's light pattern, and enforce a strict curfew in the real city.

When the German bombers spotted the city lights, they decided that their instruments were giving the wrong reading, and successfully bombed the dunes. (source)

You can use a similar approach. Build a decoy in an expendable location.

• This also happened in Britain during the Blitz: en.wikipedia.org/wiki/Starfish_site – Kyyshak Jun 4 at 11:07
• The bombing had to be overnight because of the anti-aircraft and air defense. Without this threat, the luftwaffe had bombed in daylight. – Rekesoft Jun 4 at 14:02
• @Sudix, added the source in my edit – L.Dutch Jun 5 at 10:26
• In a daylight raid the Luftwaffe would deploy dive bombers like the Stuka which are a. far more accurate and B. get so close that a light pattern won't disguise the building. – Borgh Jun 5 at 11:05
• My grandfather, having been a soldier on the German side, told me when I was a little kid how they would build replicas of installations (maybe even entire small villages) out of cardboard, slightly off the track and in a harmless location. The real location would be camouflaged (paint, nets, foliage, etc.) Result: cardboard blown to smithereens. Seems to have worked on both sides :-) (He absolutely loved model trains with all the attending model scenery, which led to that discussion.) – fr13d Jun 6 at 10:01

As described, the French lose

Bridging the lack of maps like lightning

Ok, we have pilots that have no suitable airmap. What do they do? They launch some planes and start to map the city at once. As there is no anti-air available for the French, they can fly over the city and make photos. How? Because Paris is well within the 2000 km range of the Germans Ardo AR 235 even if they would launch their recon Aircrafts in eastern Prussia! Assuming they launch in Cologne 400 km away, the French have not one week but 30 minutes before the first recon-bomber is over Notre Dame and will start to map the area. After two hours of uninterrupted photography (again, no anti-air), it turns around and lands 30 minutes later. Two hours after the landing, the High Command is not only in possession of aerial photos of all of Paris, they also are just hours away from having the best map of Paris that ever existed, showing the exact location of Notre Dame and any attempt to cover it up. Making a rough map with the river and the location of all the churches is a manner of hours now. How do they look? Well, this is a REAL aerial photo of 1943:

The next morning the air bombing can and will start, six days ahead of the plans of the Parisians, no longer stopped by lacking maps.

The Anti Air vulnerability

"A week may pass before the first bombers arrive, and anti-air weaponry will be available within two weeks of the first arrivals." - OP

As established already, the one week time to prepare the defense is a vast overestimation by the French and only feasible if the Luftwaffe has some odd lack of bombs available and needs to reroute or acquire extra bombs from other battlefields to crush Paris. Anyway, the Luftwaffe has total air dominance for two to three weeks.

Without an anti-aircraft supplement in place, now there is little to nothing the French can do between Ardo's taking photos and mapping the city and dropping occasional bombs. With the lack of any air defense, once the bombers come, there is nothing to keep the bombers away from the town and use daylight to aid in their bombing. As a result, the bombers come in broad daylight and can use the Seine and other buildings alone to estimate where to bomb to hit the one building they are interested in. See the large park north (Les Jardines des Halles) and the block of houses next to the park (Jardin des Plantes) SE? Notre Dame is where a line south from the park's east side would meet the elongation of the north wall of the university. It's in the elongation of the eastern island's edge between the first and second bridge on the second. Making a good aerial description of the target area is trivial once the photos were made.

Or they could just go to indiscriminate bombing of the inner city islands, leveling everything: Even the most useless map made from tourist photos shows that the cathedral stands on the south-eastern corner of an island, and bombing the whole island into smithereens would surely also hit that area. Hell, even with just tourist guides to guide the pilots, they might fathom a simple map: the guides and history books tell of other related landmarks like the smaller Saint Chapelle (yellow) 440 meters to the East-North-East, right next to the Palais de la Justice (green). And that Notre Dame is directly next to the huge complex of the Hôtel-Dieu de Paris (blue).

And here with more area around to show how distinct the area is:

Disguising or protecting Notre Dame is not an option anymore at this point.

Flipping the Eagle

Having no chance to cover up their Lady, the French have only one option left in the short hours: Deny their enemy the victory of destroying the church by evacuating everything from it they can and then destroying it themselves. There is no chance they can hide or protect an area about 1000 by 300 meters that is - conveniently for air attacks - right in the middle of a river.

Short of leveling the whole island and filling in the river, nothing can permanently disguise the position of Notre Dame. And nothing short of burning Paris to the ground can temporarily cover the whole city in so much smoke that it blocks aerial photography.

• Nah, there is one technicality with which the french could win this. OP said that THE FRENCH don't have anti-air, but get help from the allies. Aka. Paris has a constant patrol of British/US interceptor aircraft – Hobbamok Jun 5 at 12:03
• @Hobbamok "A week may pass before the first bombers arrive, [a vast miscalculation of the French] and anti-air weaponry will be available within two weeks of the first arrivals." - so about 3 weeks of total air domination of the Luftwaffe – Trish Jun 5 at 12:13
• I'm banking on fighters not counting as anti-air weaponry. But again, it's a technicality, and if it doesn't count your answer is the result – Hobbamok Jun 5 at 12:16
• @Hobbamok The typical French Caudron C.714 had a service ceiling of 9100 m - the Ardo could just coast along 900 meters over them and outfly them. the Dewoitine D.520C.1 could reach the Ardo's ceiling, but the Ardo was still faster... – Trish Jun 5 at 12:34
• @Trish I wouldn't call C714 typical. The design was mostly a redesign of acrobatic planes to fighters that didn't lead to a reasonable construction. Only Polish pilots decided to continue using it when the flaws of the construction were discovered (very soon all planes were formally grounded but Poles decided to fly them anyway as they had no better option at that time - other than staying idle on the ground). – Ister Jun 5 at 17:47

1. Surround the building with protective scaffolds / drapes. Here Lady Liberty stands in for Notre Dame.

2. Erect concrete walls around the entirety of the building in a ziggurat-type stepped terrace.

3. Cover horizontal parts of ziggurat with dirt. Plant grass. Install bathrooms and a snack bar at the top.

4. The citizenry can use this structure as a park while it stands. If bombers come they will see a park. If they bomb the park, the concrete bunker will protect the cathedral below.

5. 150 years later, the cathedral is still underneath the ziggurat which was never taken down. There are windows to allow sunlight to shine down inside in long rays. People like their pyramid park, and they like their cathedral protected under it.

• @MichaelSeifert Perfect for a waterslide ending in the river! – Willk Jun 4 at 16:04
• Is it possible to build something of that nature in 1 week? – Nuclear Wang Jun 4 at 17:21
• @NuclearWang It would be a miracle if something like that could be built in 1 year. – Shufflepants Jun 4 at 18:06
• @Shufflepants Frankly, it would be a wonder of the world and one of the truly great projects of humanity if it was done in a decade. The Great Pyramid of Khufu took about 20, but while it is half again as tall as The Ziggurat of Paris would have to be, its construction is also somewhat simpler, in that it doesn't need a Notre Dame-shaped cavity inside it. – Williham Totland Jun 4 at 19:40
• If bombers come they will see a park I have to question that seeing this brand new pyramid isn't simply going to tip them off. E.g. "Where's this damned cathedral, ja? - I don't know, but say Manfred, what's this giant pyramid over there?" – AmiralPatate Jun 5 at 6:02

Hide the thing pilots could know where Notre Dame should be. The islands. Bomb operator didn't looked at maps. They looked at ground below them. So the pilot would go along the river and bombardier would open the hatch when he would spot the first island. So just cover one canal with fake buildings and create fake one earlier alongside river.

• what abotu the versy distinct buildings right next to Notre Dame? – Trish Jun 5 at 7:43
• @Trish Right next are hosues. Which look from above just like any other houses. If you have in mind Louvre then, again, paint some roofs on the courtyard – SZCZERZO KŁY Jun 5 at 7:49
• You are ways of: look at the aerial photo from 1943 I put in my answer. You are looking at a gathering of some of the largest houses in the area. Even filling in one of both of the canals won't make these large, coherent houses with large courtyards not stand out among the mostly tiny roofs in paris. – Trish Jun 5 at 7:56
• Heck, even the mere existence of the Louvre and historical texts are a dead giveaway to the location of the cathedral island. – Trish Jun 5 at 7:59
• That's a great idea for a pilot - let's go the longest possible way to give as much time for AA guns to shoot as possible. German is to the RIGHT. Going from left to use Champs-Élysées as a navigation point would expose the planes to unnecessary fire. – SZCZERZO KŁY Jun 5 at 8:14

Cause a great fire and use the smoke to obscure the target.

Potential problems with this strategy would be:

• A local cloud of smoke would be a dead giveaway of the cathedrals location
• A city-wide cloud would be very difficult to maintain
• Uncontrolled fire could cause the cathedral to be harmed
• The weather may cause the smoke to be ineffective
• A great fire requires great fuel
• I also wonder if assuming enough manpower can be drafted it might be possible to construct a decoy cathedral elsewhere, doesn't need to even be all that detailed as it's trying to fool pilots that are a thousand feet away or more. Something like wooden framing covered with cloth sheets painted to give the illusion of the architectural detail of the building could possibly work, sure a cathedral is a little larger but tactics like this have been used to create decoys of whole divisions of tanks and such to fool reconnaissance etc. – MttJocy Jun 4 at 9:56
• @MttJocy Creating a flimsy dummy cathedral like that could be feasible in a short time frame. Widening the river to accommodate a new big fake island that to put it on, not so much. The Germans might not have maps, but they will know that the cathedral is located in the middle of the river Seine. – Michael MacAskill Jun 5 at 3:54
• Hmm, fire... Couldn't we use the cathedral itself for that ?? – Hobbamok Jun 5 at 11:58
• While requiring major effort, this is the only thing that will work of the yet mentioned ideas. – infinitezero Jun 5 at 12:15
• @MttJocy If you don't manage to hide to real cathedral, the Germans could just bomb both the real and the decoy. Or, if they pick one at random, they still have a 50% chance of hitting the real one. That is, if they can't distinguish between the real one and the decoy. – Abigail Jun 6 at 0:20

Strategically, it's highly unlikely that the Luftwaffe would want to bomb Notre Dame anyway. Landmarks offered pilots a critical advantage that is so obvious, it's in the name: They Mark the Land. With nothing to support actual location pinpointing, a key strategy in World War II was to spare certain parts of the city so they could find the more strategic parts of the city.

When the allies were bombing Munich, pilots and Bombadiers were under very strict orders to not bomb exactly four buildings in the central part of the city (at least one was a church). Through out the entire war, only one plane damaged the four buildings, and that was because these buildings were easily identified from the air, and configured in a square shape... If they remained standing, allied planes could approach Munich and use the four structures as a compass rose that guided them to the more important factories and infrastructure necessary to the war effort. Of the four buildings 3 escaped the war with no damage, and one only had minor damage when an allied plane flew too low and knocked the top of the spire to the ground. To give you an idea of the damage inflicted, Munich today is still uncovering unexploded WWII ordinances and the Palace of the Bavarian kings, which is one street over from the town center, is still being reconstructed to it's pre-war state... a project that is expected to conclude sometime around 2040!

Although I'm not sure on the matter, it's highly likely that Notre Dame's distinct architecture and central location would serve a similar purpose, allowing German Pilots to direction find there way to important targets.

• my points of choice to not bomb in Paris would be: Eiffel tower, Louvre, Arc de Triomphe - forming a triangle around the center. The Avenue de la grande Armee forms a good ENE axis in Paris, extendable by the Palais de Luxembourg into a trapezoid, the south-eastern corner formed by the compass-rose PdL. – Trish Jun 5 at 16:18
• Considering the Notre Dame is on one of the two island in the Seine, with the island themselves being pretty impressive landmarks, flattening the Notre Dame isn't a major loss as far as navigational landmarks go. – Abigail Jun 6 at 0:23

Since your entire premise is ridiculous [aka unsolveable] (especially the "no maps" with photography existent...) I'll save the cathedral with some technicality:

together with help from the Allies, prevent the bombers from spotting the cathedral?

ENHANCE

together with help from the Allies

And there you go.

No German bomber can even spot the cathedral (not even hit it), if you have British and US Interceptors swarming France 24/7 (like the beginning of '45).

Yes, ground-support Anti Air would be nice, but the french can't provide that apparently so the fighters alone would have to do (and will do, assuming late-war setups).

If that counts as "anti air available", then the cathedral is 100% lost, as @Trish laid out with much detail. All the other stuff other people mention here won't work, because they assume either night-raids or super-high-flying bombers, neither of which will be happening here.

If by "no maps" you mean that Germany magically can't produce anything maplike, they'll just level Paris and call it a day, they'll hit the cathedral eventually. Or use StuKas, with which they can hit it with bare eyes.

Similar to SZCZERZO KŁY: While these events were occurring in Paris, Boeing built a fake neighborhood on the roof of an airplane plant.

https://99percentinvisible.org/article/prop-town-fake-rooftop-suburb-hid-whole-wwii-airplane-factory/

This neighborhood (made by a Hollywood set designer) was half the size of Notre Dame's island, but you'd only need to cover the cathedral and some of its surroundings, which could be much smaller.

• So the bomber crews don't find ANY cathedral the first time, go back. The commander concludes that it's hidden and Germany simply levels Paris. [But nice Idea and example] – Hobbamok Jun 5 at 12:13
• I think you'd also have to create a fake cathedral on the other island, and ideally cover the river to the south of the real island with more fake buildings. – Robin Bennett Jun 5 at 13:01
• @RobinBennett Problem there is that the other island is dangerously close given the inaccuracy of unguided air dropped bombs, ran into a similar issues with my own thinking that you would need to create a decoy as without one any attempt to obscure the target is going to look suspicious. But yeah ideally making full use of scenario fulfilment by giving them a target so they don't look too close at anything else going on nearby as they are too focused on the mission would make sense. A flimsy decoy of a building is pretty easy but rivers not so much so might have to just risk it. – MttJocy Jun 7 at 9:38

The french (we :)) already did it during the first world war.

At the end of WWI, to avoid bombardment on Paris, the french did a "fake Paris" in countryside, reproducting the great landmarks of the city and mimicking the night life of Paris.

It was ALL about the lights on grounds, because bombers were coming at night to avoid anti air defenses. I don't think this perfectly answer your question because of this, but maybe it could give you some cool ideas.