I cannot remember the explanation. But it could be a lump of ore in the Sea very deep off the continental shelf. Otherwise I can't remember what it was, need to look it up. But there was a natural explanation to the magnetism. It weren't Atlantis and the crab people.

There are Islands off Finland in the Baltic sea. They have had a huge iron deposit and again compasses supposedly go spinning near there. There are few other points and on land with it. Normally ore, sometimes other phenomenon of isolated valleys.

In those triangles they are the subjects of weather and it causes compasses and electronics to fritz in storms. Lightning, storms, etc. But regardless there is the phenomenon of those readings outside of storms being problematic. It's a contact point of the various currents streams warm, cold generating weather. Need to look it up fully.

The missing ships, and planes. Explained as a point of hurricanes, and in the South China sea, typhoons. Both are also the subjects of piracy for much much longer.

It isn't a wormhole where Indiana Jones goes back in time. Another conspiracy, apart from the crab people, is the polar flip, and shift. This is otherwise fantasy.