Likelihood of a large magnetic island remaining undetected until the early 20th century

Question

In Edgar Rice Burroughs' The Land That Time Forgot, the crew of a captured German U-Boat discovers a large island in polar waters somewhere between South America and New Zealand called Caspak. Among the unique qualities of Caspak is that it is highly magnetic, so much so that it was able to render the sub's magnetic compass useless for several days. This story takes place in 1916.

Is it at all likely that an island with such powerful magnetic effects could remain undetected until the early 20th century? Obviously, Antarctic waters were less well traveled than other parts of the globe, but they were still trawled by fishing and whaling vessels, and 1916 is only a few years after the overland expeditions to the south pole. But perhaps the traffic in this area (close to the oceanic pole of inaccessibility) was so light that it could be overlooked.

It will be necessary to consider the size of the compass-distorting affects here. A WWI German U-Boat like the U-31 class had a cruising speed of 8 knots on the surface. Assuming that the crew experienced this magnetic distortion for five days, and the submarine was travelling continuously, this translates into an area of effect with a radius of 960 miles.

So, is this a credible scenario? I don't know enough about the nautical traffic in the area during that time in history to say. Were there active efforts to identify such magnetic anomalies that would likely have detected a place like Caspak? If the scenario is not credible, where else might a similar island be located? Would such a large magnetic anomaly be detectable, even if only faintly, by magnetic surveys on nearby land such as New Zealand or the Pacific Coast of South America?

N.b. I am asking this in Worldbuilding instead of Science Fiction & Fantasy or Earth Sciences because I am considering how to adapt this public domain setting into one of my stories, and answering it involves historical as well as scientific considerations.

Answer 1

An interesting bit of information to know is the existence of Canada's Landsat Island. The island is named for the fact that it was discovered only after analysis of photographic images from the Landsat 1 satellite system. This discovery happened in 1976.

Now, Landsat Island is tiny, basically a glorified rock. But it went unnoticed by all ships and aircraft passing by Labrador for all that time. Would it be surprising that some larger island further from large land masses went unnoticed prior to planes, ship-borne radar, and satellites? It seems entirely plausible.

As for a magnetic disturbance being the cause of the unnoticed island, it is less certain. I prefer the simpler explanation that no one ever bothered to sail through that patch of water.

Answer 2

First off, I had The Land That Time Forgot arrive from Amazon literally 30 minutes ago, so I'm keen on this question (no spoiler alert!?!?!) though I haven't read the book yet, obviously.

Second, the answer to your question is that this is definitely not plausible. Its hard to overestimate the curiosity of those scientific and exploration types of the 18th and 19th century. Edmund Halley (of comet fame) went to the South Atlantic in two trips from 1698 to 1700 specifically to record compass variations. He published a book of his records (for use by mariners) called General Chart of the Variation of the Compass, and I believe that in gathering the data for this book, such a magnetic anomaly would have been noted.

If he didn't discover it, Yves Kerguelen discovered the Kerguelen Archipelago in 1771 and Captain Cook sailed along most of the Antarctic Circle between South Africa and the International Date Line in 1772-1774.

Finally, by the early 1800s, there were MANY whalers and sealers plying their trade in those regions. A whaling camp was set up in 1764 in the Falklands, seal hunting began at South Georgia in 1786, whalers reached Kerguelen in 1800, Bouvet in 1804, Macquarie (probably the closest land to the Land that Time Forgot) in 1810. And there were a LOT of whalers down there, some 700 ships at in the 1840s from Nantucket, New London, and New Bedford.

So in conclusion, someone would have found it, by the early 1800s, for sure.

Is there anywhere where no-one would have found it? The oceanic pole of inaccessibility is in the south Pacific, closer to Chile at 48S 123W. This is where Lovecraft placed R'lyeh. Also, I would recommend the North Pacific Gyre, either on the eastern end, halfway between Hawaii and Japan or the western end halfway between Hawaii and San Francisco. Nowadays the great circle route to/from Hawaii goes right through this, but before the age of steam ships, these were doldrums like the Sargasso Sea, so sailing ships would have strenuously avoided. There are also no islands at this latitude (~30N) other than the Hawaiian chain, so there is lots of empty room left in the vast North Pacific.

Answer 3

The first thing to note is that your compass is wrong.

You're trying to hide your island in what is currently a relatively clean bit of magnetic field, though you'd need to check a map that was correct for the period to see where the Magnetic South Pole was at the time. The closer to the pole the easier your island is going to be to hide.

Luckily you've picked the right end of the world, the south pole is all at sea and accessible from a submarine. There are also strings of islands of the southern end of New Zealand, match those to a highly mobile magnetic pole and approximate mapping techniques and you could probably talk your way out of an island being properly mapped reasonably easily.

However the more isolated your location the more likely it is that an island there would be noticed.