A large triangle-shaped area in the North Atlantic Ocean where strange events are said to occur. The three corners of the triangle are considered to be at Bermuda, Miami, and San Juan, Puerto Rico.
Many planes, ships, and people are said to have vanished here. Who's behind it? Is it aliens? Raiders from Atlantis? Kidnappers from the future? Ocean Dracula?
Honestly, it's a big area with lots of wacky currents where a ship could sink without anyone knowing what happened to it. The number of ships and planes that have gone missing in that area isn't particularly greater, proportionately, than in any other part of the ocean, but we're a pattern-loving species, so we often see interesting patterns in interesting locations.
Besides, everyone knows it's impossible for something to disappear without a tra