The five men onboard the Titan sub that disappeared on Sunday while on a trip to the depths of the Atlantic Ocean to visit the wreck of the Titanic have been declared dead.
The oxygen on board the Titan submersible lost beneath the Atlantic ran out shortly after midday on Thursday - but what happens when there is no breathable air left?
The Titan submersible that has gone missing while on a descent to the wreck of the Titanic got lost on a trip last year, with a journalist who was on board saying: "There's no backup, there's no escape pod - it's get to the surface or die."