A tourist submarine that was exploring the wreckage of the Titanic has gone missing and there are fears that it has imploded thousands of feet underwater. The sub was carrying five people, including British billionaire Hamish Harding and French explorer Paul-Henri Nargeolet.

Experienced diver G. Michael Harris, who may know some of the occupants, said he is not optimistic about their rescue and believes there is nothing the US Navy can do. The sub last "pinged" above the Titanic wreckage on Sunday before losing signal. It is believed that the sub has enough oxygen to last until Thursday, but an implosion would mean any hope is already lost.