Two prevailing theories:
1. Sub imploded about two hours into the mission. 2. There was a fire on board.
There were many problems with this vessel:
- Supporting staff was all under age 30 with little or no sub experience. - The 5-inch-thick carbon-fiber material was never fully tested at 4000 meters depth. - There were no gas safety monitors on board to check O2 and other gas levels. - There were no extra oxygen tanks and masks on board. - 96-hour life support system was never tested with a full crew on board. - The hatch could only be opened from the outside. - There was no emergency transponder in the vessel. - PlayStation joystick to control the vessel was wireless and there was no back-up system if it failed. - Communications system was not voice capable and had a tendency to black out.
So basically, it was an underwater coffin Give credit where its due, that is a list that comes from Sub Brief, the excellent YouTube channel run by an ex US submariner:
The Titan Tragedy