Pressurized air was then released into the vessel to build up enough pressure so it would be possible to open two hatches on the underside, while keeping water out. This meant that air pressure inside the submarine had to equal water pressure at diving depth, exposing the crew to high pressure, making them susceptible to decompression sickness, which was unknown at the time. To surface, more of the pressurized air was used to empty the ballast tanks of water.