When they reach the end of the tunnel, the two of them end up in a completely submerged room, a room designed only for scuba divers and not for anyone who wants to swim there without any equipment, which they found out quickly as they swim to the celling and find no surface up above.
This, however, did little to deter them from turning back, as their curiosity led them to the remaining building in the depths, wondering what sort of things they might find there.