It was the submersible that promised passengers the trip of a lifetime. A chance to descend 3,800m (12,500ft) to the Atlantic depths to visit the wreck of the Titanic.

But last year, a dive by Oceangate’s Titan sub went tragically wrong. The vessel suffered a catastrophic failure as it neared the sea floor, killing all five people onboard.

The US Coast Guard is holding a public hearing on 16 September to examine why the disaster happened, from the sub’s unconventional design to ignored safety warnings and the lack of regulation in the deep.

Titan began its descent beneath the waves on the morning of 18 June 2023.

On board were Oceangate’s CEO Stockton Rush, British explorer Hamish Harding, veteran French diver Paul Henri Nargeolet, the British-Pakistani businessman Shahzada Dawood and his 19-year-old son Suleman.

Later that day, after the craft failed to resurface, the US Coast Guard was notified, sparking a vast search and rescue operation.

The world watched and waited for news of the missing sub. But on 22 June, wreckage was discovered about 500m (1,600ft) from Titanic’s bow. Titan had imploded just one hour and 45 minutes into the dive.

These are five key questions that still need to be answered.