Throughout history, scientists have relied on a number of specialized tools to measure, map, and view the ocean's depths. These are a few highlights. One of the first instruments used to investigate the sea bottom was the sounding weight. Viking sailors took measurements of ocean depth and sampled seafloor sediments with this device, which consisted of a lead weight with a hollow bottom attached to a line. Once the weight reached the ocean bottom and collected a sample of the seabed, the line was hauled back onboard ship and measured in the distance between a sailor's outstretched arms - a 1.83-meter (6 ft) unit called a fathom. This term is still used today for nautical depth. Cornelius van Drebel, a Dutch inventor, is usually credited with building the first submarine. His underwater vessel consisted of a wooden frame sheathed in leather. Oars extending out the sides propelled the craft through the water, at depths up to 4.6 meters (15 ft). The oar openings were sealed with tight-fitting leather flaps. Drebel tested the sub in the Thames River in England between 1620 and 1624. King James I is said to have taken a short ride in the craft. From 1872 to 1876, a landmark ocean study was undertaken by British scientists aboard HMS Challenger, a sailing vessel that was redesigned into a laboratory ship. The Challenger expedition covered 127,653 kilometers (68,890 nautical miles) and is credited with providing the first real view of major seafloor features such as the deep ocean basins. The researchers used wire-line soundings to determine depths and collected hundreds of water, sediment, and biological samples from all the oceans except the Arctic. They discovered more than 4,700 new species of marine life, including deep-sea organisms. Deep-sea exploration advanced dramatically in the 1900s with a series of inventions, ranging from sonar - a system for detecting the presence of objects underwater through the use of sound - to manned deep-diving submersibles such as Alvin. Operated by the Woods Hole Oceanographic Institution, Alvin can carry a crew of three people to depths of 4,000 meters (13,124 ft). The sub is equipped with lights, cameras, computers, and highly maneuverable arms for collecting samples in the darkness of the ocean's depths. However, the voyage to the ocean floor takes time in a submersible - it's a cold, four-hour roundtrip to hydrothermal vent sites 2.4 kilometers (1.5 mi) below the surface - and scientists are working to find ways to observe this extreme environment without being in it. With the expanded use of fiber optics, satellites, and remote-controlled robots, scientists someday may explore the deep sea from a computer screen in the lab rather than out of a porthole.   Getting to the Bottom of It: The Ocean Because of the tremendous pressure, the depth to which a diver can descend without special equipment is severely limited. The deepest recorded dive by a skin diver is 127 meters (417 ft). The deepest recorded dive by a scuba diver is not much farther, at 145 meters (475 ft). Revolutionary new diving suits, such as the "jimsuit," enable divers to reach depths up to about 600 meters (2,000 ft). Some suits feature thruster packs that can boost a diver to different locations underwater. To explore even greater depths, deep-sea explorers must rely on specially constructed steel chambers to protect them. In 1934, American oceanographer William Beebe and engineer Otis Barton were lowered to about 1,000 meters (3,280 ft) in a round steel chamber called a bathysphere, which was attached to a ship on the surface by a long cable. During the dive, Beebe peered out of a porthole and reported his observations by telephone to a colleague, Miss Hollister, who was on the surface. In 1948, Swiss physicist Auguste Piccard began testing a much deeper-diving vessel he invented called the bathyscaphe. (This word is derived from the Greek words bathos -"deep" and scaphos -"ship".) On an unpiloted dive in the Cape Verde Islands, his invention, named FNRS 2, successfully withstood the pressure on it at 1,402 meters (4,600 ft), but its float was severely damaged by heavy waves after the dive. In the 1950s, Jacques Piccard joined his father in building new and improved bathyscaphes including Trieste, which dived to 3,139 meters (10,300 ft) in field trials. The U.S. Navy acquired Trieste in 1958 and equipped it with a new cabin to enable it to reach deep ocean trenches. In 1960, Jacques Piccard and Navy Lieutenant Donald Walsh descended in Trieste to the deepest known point on Earth - the Challenger Deep in the Mariana Trench. The two men made the deepest dive in history: 10,915 meters (35,810 ft). Today, scientists are making exciting discoveries about the ocean floor, thanks to deep-sea submersibles such as Alvin. Operated by the Woods Hole Oceanographic Institution in Massachusetts, this three-person submarine made its first dive in 1964. Since then, Alvin has made more than 3,000 dives to average depths of 1,829 meters (6,000 ft). Alvin has conducted a wide variety of research missions, from discovering giant tubeworms on the Pacific Ocean floor near the Galápagos Islands, to locating the wreck of HMS Titanic in the Atlantic Ocean. Alvin has met some amazing life on its travels, but so far it has not encountered one of the deep sea's most mysterious inhabitants - the elusive giant squid. However, Alvin once was attacked by a swordfish, which became trapped between two pieces of the sub's fiberglass skin. The fish was brought back to the surface and cooked for dinner!