First discovery of the hadal zone

The hadal zone is any point in the ocean that stretches more than 6,000 metres (19,685 feet) below the surface. The discovery that the ocean reached such depths was first made on 23 March 1875 by the British round-the-world HMS Challenger expedition (1873–76), which, using a weighted rope, unexpectedly plumbed a depth of 4,475 fathoms, or 8,184 m (26,850 ft), south of the Mariana Islands in the Pacific Ocean. This expedition would go on to lend its name to the deepest point in the ocean, the 10,935-m (36,876-ft) Challenger Deep situated at the bottom of the Mariana Trench.

The first hadal organisms were collected in 1901 by scientists on board the Princess Alice II, a steel schooner built for Prince Albert of Monaco as a marine research vessel, which trawled deep-sea specimens of Echiuroidea (spoon worms), Asteroidea (starfish), Ophiuroidea (brittlestars) and demersal (bottom-dwelling) fish from 6,035 metres (19,800 feet) in the Zeleniy Mys Trough south-west of Cape Verde in the North Atlantic Ocean.

The first photographs of the hadal zone were taken in 1956 by the French oceanographer Jacques Cousteau at c. 8,000 m (c. 26,250 ft) on the crewed submersible Calypso, in the North Atlantic's Romanche Trough.