The Drawbacks of Manned Submersibles
But submersibles have their limitations. For one thing, they are designed to travel at speeds of 1.6 to 5.6 kilometers (1 to 3.12 miles) per hour. This slow speed is sufficient for examining a small site, but it restricts a vehicle's range. Submersibles also take a long time to get to and from the sea floor. A typical dive may last six to eight hours, most of which is spent in vertical movement. Scientists become tired and uncomfortable after sitting in the cold, cramped pressure hull for several hours. Furthermore, the power supply of most submersibles is limited because they depend on batteries.
Expense is another limiting factor. Submersibles cost far too much to maintain and operate to simply go out and scour the ocean bottom, looking for interesting things. A submersible must be transported to a dive site aboard a surface ship, and another ship must first scout a proposed site with towed cameras and other instruments to make sure it is worth investigating firsthand. For example, a series of hydrothermal vents that Alvin visited in 1977 in the Pacific Ocean near the Galapagos Islands were first discovered by a towed temperature sensor, then double-checked with sonar, towed cameras, and laboratory analysis of water samples. Only then was the decision made to send Alvin down. Considering the work and expense involved with undersea exploration, it is not surprising that oceanographers often return to sites they have visited previously in order to study them more intensively. They are reluctant to visit a new site and come back with nothing.