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.