The Titan sub, operated by OceanGate, suffered a "catastrophic implosion".
The disappearance and "catastrophic implosion" of the Titan sub, which was taking five tourists to see the wreckage of the Titanic, was one of the most unsettling events in recent years. The incident raised questions about the vessel's safety and on the great depths people go to see what remains of a ship that sank in 1912. There are 10 sea vessels that can reach the depths of 4,000 metres or greater and Titan, owned by OceanGate, was the only one that wasn't certified by any regulatory body, according to the BBC.
OceanGate had been warned by both industry experts and one of its senior employees that the sub might be unsafe.
Still, a visit to the Titanic wreck at a depth of 3,700 metres is one of the most popular and dangerous expeditions in the world. Less than 250 people have personally viewed the wreckage that was discovered 75 years after the sinking of the Titanic.
One of them is filmmaker James Cameron whose interest in shipwrecks served as a motivation for him to make 'Titanic'. It won the Best Picture Oscars in 1998. The director has taken 33 dives himself to the wreckage of the Titanic, according to USA Today.
In 1995, he was on a third dive with submersible pilot Dr Anatoly Sagalevich and a Russian engineer. They unexpectedly encountered a sandstorm on the ocean floor.
"Anatoly said 'Oh, no', something you never want to hear a pilot say, and we locked eyes for a second," Mr Cameron recalled in his 2009 biography 'The Futurist'.
The strong currents sapped the sub's power supply and they were almost out of batteries. The crew aborted the dive but instead of going to the ocean floor, the vessel stopped rising and sank back to the ocean floor.
Mr Cameron and his crew waited for half an hour before trying again, but the sub stopped at 80 feet. The water current, however, kept blowing them away from the Titanic. On their third attempt, the crew saw the vessel rise from the 80-feet mark and away from the near-freezing temperature, breaking the surface five hours later.
There are some previous expeditions too experienced close calls that could have cost the crew their lives.
In 1991, Canadian undersea physician Dr Joe MacInnis undertook an expedition, not just to carry out biological and geological studies but also capture the wreckages in IMAX film. Two Russian submersibles made 17 dives during the entire expedition, but the last one hit a snag.
After shooting the wreckage, the researchers tried to lift the platform but realised it was stuck. A second submersible that arrived to help them saw the left landing skid had slipped under a mass of wires. It then gave them the directions on how to manoeuvre their way out of the tangle.
"We had that second pilot, that second sub, self-rescue capability. So we were very fortunate," Ms MacInnis told Times Radio.
Michael Guillen also decided to visit the Titanic wreck in 2000, hoping to become the first reporter in 88 years to do so. But as his sub crossed the debris field between the ship's front section and the stern, Mr Guillen found they were speeding up. Like Mr Cameron's crew, they were caught in one of the deep sea's unpredictable currents.
"A split-second later, our sub slammed into the Titanic's propeller. I felt the shock of the collision; shards of reddish, rusty debris showered down on our submersible, obscuring my view through the porthole," Mr Guillen later wrote in his book 'Believing is Seeing'.
They spent an hour stuck in the great depth before their sub started rising again.