Titanic sub: What are the key challenges in the search mission?

The US Coast Guard says a search covering 10,000 square miles (26,000 square kilometres) has turned up no signs of the missing submersible [Handout: OceanGate Expeditions via AFP]

A submersible vessel taking five tourists on a deep ocean journey to view the wreckage of the Titanic went missing on Sunday, triggering an enormous search and rescue effort.

Here is a look at the situation as rescuers race against time to search an area about half the size of Belgium for the submersible, which could be on the ocean floor or bobbing on the surface, with the tourists bolted in from the outside.