NASA tests cellphone-sized underwater robots for future ocean world missions (video) 

 Engineers at NASA's Jet Propulsion Laboratory recently tested prototypes of miniature, self-propelled robots designed to explore the hidden oceans of distant moons like Europa, Jupiter's icy moon. 

https://cdn.mos.cms.futurecdn.net/ujAYhx4tNN6tmZKpeUC6HA.jpg

https://www.space.com/space-exploration/missions/nasa-tests-cellphone-sized-underwater-robots-for-future-ocean-world-missions-video