This is a bit of an addendum to a previous question, shown here. (How to Cause a Hydrosphere Apocalypse Part II The Great Flood). I like the rain of ice from a dying moon, answer and I have decided to use the world in part two as a springboard for a setting in a novel.

But I have run into a tad bit of a snag.

The wiki for the film Waterworld, (a major bit of inspiration) says that the global flood added 25,000 ft to the global sea level. Now, this presents a couple problem for the story I wish to tell.

See the locals are air breathers capable of holding their breath for 30 minutes. I want them to be able to reach the sea bed, (at least for the shallower parts) and scavenge for parts and one set piece I really wanted to use is the ruins of a flooded city with the tops of sky scrapers poking out of the waves, inspired by this image by Slava Vidyapin. flooded city

Now the shallowest sea bed being 4.7 miles down presents an understandable problem to all those requirements.

But then it hit me. Erosion.

The flood that will swallow this particular world isn’t coming up from a rising sea, it’s coming from above in a rain of ice and water. And not just for 40 days and nights but for centuries even millennia. And usually such rains can wash away the land and thus maybe make the resulting sea that much shallower. But I wanted the opinions of those who know better than I.

So, Would a thousand years of rain and icy meteors lead to enough erosion to ensure a shallow sea water world?

  • 1
    $\begingroup$ A thousand years of drizzle won't do much erosion; for example, the island of Great Britain has been rainy for a lot more than a thousand years, and it still has sort-of mountains. So, we need to know how hard it rains. (But please consider that mass is mass, and having a lot of mass coming down from outer space will boil the oceans and melt the crust, with all the dire consequences. It doesn't matter that the gigantic mass is water, or ice, or liquid nitrogen.) (Ah, and that silly movie Waterworld is in no way "science-based". It was intended as a mindless pass-time.) $\endgroup$
    – AlexP
    Apr 1, 2022 at 21:54
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    $\begingroup$ You need to do away with both continental plates and volcanism. $\endgroup$
    – Alexander
    Apr 1, 2022 at 22:31
  • $\begingroup$ You would need a large mix of conditions: no new plate tectonics, constant torrential storms for more like millions of years, and some kind of corrosive (like acid rain) to break down the rock. Even then, there would likely be a little land left. $\endgroup$
    – DWKraus
    Apr 2, 2022 at 0:57
  • $\begingroup$ There are similar questions with many answers. Try this one worldbuilding.stackexchange.com/questions/201259/… $\endgroup$
    – DWKraus
    Apr 2, 2022 at 1:00
  • $\begingroup$ I mean ok why not have handwavy bits like some places from the former ground had broken off and risen up due to explosive volcanic activity so there would be columns of volcanic formations like 4-ish miles high. Since some cities are literally floated, I guess they could have broken off the ground, and floated until it landed like somewhere on the side of everest? I mean at five miles high, everest will still have a little dry land if the sea is 4.7 mi off the former sea level. $\endgroup$
    – Harry Mu
    Feb 20, 2023 at 19:52

2 Answers 2


Not in a way you want.

Land tectonic plate thickness is about 100km thick, sea tectonic plate thickness is much less, about 10 km. Even if we take just the hight difference on the surface, thats still 4km.

If you manage to erode this much matter, how would city ever stay available to scavangers of any sort? Even 1m erosion will make almost all city ruins unaccessible without modern machinery. 100m erosion will make it non-existant but detectable on chemical level. 10km erosion will make it only detectable on isotope level. Doesnt matter, if it is erosion by weather or meteorites or whatever, same rule applies.

If you want water world with accessible city ruins your options are:

  • Flat planet with large ice caps. When they melt, most land is under water, softly, without destruction. Centuries - people will adapt somewhat - floating bases.

  • Flat planet and meteorite shower. Very small meteorites, none of them reaching the ground, large water content. Even then dust from them will still cover the ruins. And it will take millions of years - people will adapt fully in time. If any faster - atmosphere will overheat.

  • Some strange tectonic activity, so that all tectonic plates melt and reform at about equal thickness. So that land and sea surfaces evens out. Likely includes extreme volcanism and earthquakes, making ruins barely accessible. Will still take millions of years. People will fully adapt in time. If any faster - requires extreme energy that doesnt exist in a planet normally. Like a black hole or a large payload of antimatter.

  • Local catastrophe. Be it a large nuke or volcano, if flat planet consist of one deep sea, it could be 'filled up' with local material. It will destroy most of the ruins, but some will remain.

Anyway, consider the amount of heat your idea involves.

P.S. city ruins are useful only for a century. After that it is an ore deposit.


Rain from below.

Continents sit on thick lumps of continental crust. These are the floats that keep continents up - without them, the continents would go down.

Now, there's obviously no way on the Earth we know to say that these will all wear away all over the planet any time soon. But maybe for your planet, we can propose there is a continental crust sort of like ours, but it overlies a layer of "liquid core" with no separate "mantle" between. That liquid core is cooling, turning solid at the center, like liquid cores on planets we know - but this particular one has gotten very, very thin. So thin that the underlying solid core (which rotates relative to the crust below, like ours does on Earth) has begun to rub against the continents in places, throwing rubble back and forth between the solid continents and the solid inner core. As the rubble crashes into the roots of the continents, it wears them away and mixes them with the remaining liquid core material, which is rapidly heating from the friction.

The result we're looking to get to is that all the continents are ground down from below. They sink within your time frame mostly because this is not Earth (ours are still rebounding from the Ice Age), but we'll say they're a little less stiff for some reason.

The net outcome is a planet with a thin shell of liquid core, surrounded by a crust that has been sanded smooth, with all the terrain above evenly distributed beneath its ocean. No water needed to be added, but now everything is underwater.

  • $\begingroup$ Maybe I don't understand this answer, but I wonder if this scenario could render the planet into a magma inferno ? Wouldn't the water evaporate ? $\endgroup$
    – Goodies
    May 5, 2022 at 20:53
  • $\begingroup$ It's a purely hypothetical idea, not rooted in any science I know. I'm supposing the outer core is heating, which could definitely drive volcanism - but it's not that clear to me when. It will take time for the heat to percolate through the crust. Also, the amount of heating is limited by the melting/disintegration of crustal material, like a pot of hot water with ice on top, which limits how hot it gets. But yeah - it's hard to picture an Earthlike planet without a single volcano breaching the ocean, even before we mess with it dramatically. $\endgroup$ May 5, 2022 at 20:57

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