Sailors once measured depth by lowering a weighted rope, a practice known as sounding the sea.

The longer version

Depth was once measured by lowering a weighted line until the bottom was struck and reading off how much had run out. A hollowed cavity in the weight brought up a sample, allowing crews to tell sand from mud or shell and judge whether holding ground looked promising.

Reading a line required slowing considerably, so the method was used carefully rather than constantly, with results recorded beside position, wind and weather. Electronic pulses now do the work in an instant, yet older units of depth still reflect how those ropes were marked. Repeated casts built the early picture of ocean depth long before echo methods existed.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.