What sub debris has been found and what does it mean?

The US Coast Guard has announced that a field of debris has been found on the seabed near the wreck of the Titanic, where search teams are looking for a missing submersible.

According to dive expert David Mearns, the debris includes "a landing frame and a rear cover from the submersible".

The BBC's science correspondent, Jonathan Amos leads us through what these finding suggest.