The hole is characterized by a shallow opening to the sea and a tunnel known as the "Arch". The Arch has a ceiling with a depth of 181 feet and a bottom that falls away.
It is the Arch and the underwater tunnel in the Blue Hole that makes the site so dangerous for divers. Escaping from the tunnel can be deadly.
According to Atlas Obscura, at least 40 divers or more have been killed in the Blue Hole. Many of these divers were highly experienced.
One of the most notable deaths at the Blue Hole was that of Yuri Lipski. Lipski was a young, Russian diving instructor. He inadvertently filmed his own death as he made an uncontrolled descent. After reaching a depth of 377 feet, he could not make it back up and suffered from nitrogen narcosis. His camera and body were eventually brought back to the surface.
An article in Forbes describes how dangerous it can be to dive into the Blue Hole. First, getting to the Arch requires a descent of 185 feet. Then they need to go through the tunnel which is another 85 feet before they get to the open water.