How did early explorers determine the depth of and ocean

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.

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Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.