He says these underwater tests help astronauts figure out how much mass they need to propel themselves forward.

The crew and mission control have also experimented with communication delays, from the brief delay astronauts experience on the space station to the roughly 20-minute delay that could exist on Mars.

Crew members also study the ways their bodies react to extreme environments and changes in pressure by monitoring their heartbeats and using radar to chart their blood flow.

By the time the 14-day mission wraps Monday, the crew will have conducted a total of 52 so-called "space walks" in the sea.

At the end of the mission they will ascend to the water's surface over the course of about 16 hours, at a rate of roughly one meter per hour, so their bodies can adjust to the changes in pressure.