A drone that can help divers and snorkelers explore or reveal the location of shoals of fish is being developed by a start-up in the US.

Controlled via a smartphone app, the Fathom has an onboard camera and a turbine that directs water through holes in its shell, allowing it to dive, steer and surface.

Expected to cost about $450 (£290). it is believed to be the first underwater drone purpose-built for consumers.

It was inspired by a lake in Michigan.

Co-creator Danny Vessels said: "I had always heard rumours of horse drawn carriages that were used to transport logs across the frozen lake and had fallen through the ice," he told the BBC.

"Locals always claimed that they had gotten stuck at the bottom and are still there today."

Mr Vessels looked online for submersible drones but found only the very expensive craft used by oceanographic researchers and survey companies or drones requiring significant technical skill to assemble.

After four months working on the Fathom drone with his friends Matt Gira, Matt Webb and John Boss, he is now refining the prototype.