The first instrument used was called the sounding weight. (a weight made of lead, roughly 5kgs in weight) . In the early 1900's they began using sonar systems to detect sound from deep down and also a submarine that could contain a max of 3 people.
The early explorers determined the depth of an ocean with the help of an instrument called 'sounding weight'. The method of measuring the depth of an ocean using this instrument was called 'Depth Sounding'.
Explanation:
This instrument called 'Sounding Weight' used by the early explorers to measure the depth of the oceans was basically made up of a bell-shaped mass of lead tied to a hemp rope.
This rope had markings at an interval of 6 feet, which made one fathom. This instrument dropped into the ocean freely and the markings disappearing in the water were counted.
