|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really existed), and, as such, details contained in this article may differ from real world facts.
The Bermuda Triangle is a region in the Atlantic Ocean where numerous boats and aircraft are alleged to have mysteriously disappeared. TerrorTours, a wizarding travel agency, advertised a cruise through the Triangle which would allow vacationers to see the wrecks of ships which failed to notice the area's resident monster.