Short answers, pulled from the story.
Pierre-Simon Laplace investigated the average depth of the Atlantic Ocean circa the late 18th or early 19th century. He calculated the depth to be a value later proven quite accurate by echo-sounding measurement techniques.
Norwegian researchers Michael Sars and Georg Ossian Sars discovered the first deep-sea life forms in 1864. They obtained a sample of a stalked crinoid at a depth of 2,000 meters which challenged existing theories about life in extreme environments.
Edward Forbes claimed that diversity of life in the deep sea decreases with increasing depth and stated there could be no life in waters deeper than 1,000 fathoms. This hypothesis became known as the Abyssus theory until Michael Sars found rich deep sea fauna at that depth near Lofoten.
William Beebe and Otis Barton reached a depth of 3028 feet in 1930 while diving in the Bathysphere. During this historic dive they observed jellyfish and shrimp through a porthole.
Jacques Piccard and United States Navy Lieutenant Donald Walsh descended to the Challenger Deep in the Mariana Trench in 1960. Their dive reached 35,800 feet making it the deepest dive in history.