Vikings were among the first to measure the depth of the ocean. They used a device called a sounding weight that was lowered to the sea floor. Once it reached the bottom, a hollow chamber at the bottom of the weight collected sediment. That way, sailors could know they had reached the sea floor. When they retrieved the rope, they determined the length needed to reach the sea floor by using a man's outstretched arms as their unit of measurement.
One fathom measures 6 feet. The word comes from the Old English word faethm, which meant outstretched arms. One fathom is roughly the measurement of a person's arms outstretched, meaning the modern word stems from the ancient practice. Fathom can be used in multiples to describe deeper measurements: Five fathom equal 30 feet. If you bury a person at sea, they typically must be buried at least six fathom deep.
A shackle originally described the length of an anchor chain. It is equal to 15 fathom. This term was used when anchors were connected by linked chains or shackles rather than rope or cable. Throughout much of this time, ships were equipped with 12 shackles of chain. Until the mid-20th century, the length of chain that equaled one shackle measured 12.5 fathom. The change was made by the Royal Navy after World War II.
Divers measure sea depth in meters. Skin divers have gone as deep as 170 meters, and a diver wearing scuba equipment has descended as far as 308 meters. Divers wearing special suits called "Jim suits" or atmospheric diving suits that maintain the atmospheric pressure on land have dived as far as 600 meters below sea level.