In August 2004, the Woods Hole Oceanographic Institution (WHOI) and the National Science Foundation jointly announced a $21.6-million program that will replace the venerable manned submersible Alvin. The yet unnamed vehicle will be able to dive to 21,320 feet compared to the Alvin's 14,764 feet, a depth increase of 31%.
The new submersible will join four others in this depth class: Russia's two Mirs (20,000 feet), France's Nautile (20,000 feet), and Japan's Shinkai 6500 (21,500 feet). Also in 2007 the People's Republic of China will put a 23,000-foot-capable submersible into service. Under construction now, she will be the world's deepest-diving manned submersible.
What is important about the 20,000-foot depth? A submersible capable of reaching this depth can access 97-98% of the oceans' seafloor. Thus, only 2-3% of the seafloor is between 20,000 and 36,000 feet at the oceans' deepest point. From engineering and cost points of view, designing for this depth makes sense.