The submarine created in the early 1600's was likely based off of a row boat that had a covered top and was powered by oarsman within the boat. A virtually neutral buoyancy and snorkel air tubes held above the water by floats permitted the leather-sealed boat to remain submerged for hours on end. In order to keep the air at a reasonable quality, Drebbel used a combination of chemicals to purify the air. The outer frame was wooden covered with a greased leather seal. The submarine practiced up and down the Thames River and held twelve oarsmen along with a handful of other passengers. The foredeck was most likely downward-sloping to act as a sort of diving plane that would be submerged underwater by forward momentum similar to modern submarines. If the rowers slowed or stopped the boat would rise to the surface again. The submarine supposedly reached a depth of around 15 feet underwater and is the earliest case of a submarine that was actually built. This trip through the Thames lasted three hours, but despite being built during his time working for the British Navy, Drebbel did not get an opportunity to do anything more then test his invention. It was not until decades later that the submarine was used as anything more than an interesting idea or sideshow for a few extra coins.