Don Walsh was the first to go to the very bottom of the deepest part of the ocean in 1960 in a specially designed submarine, the Bathyscaphe Trieste. The water pressure was 800 tonnes per square inch, and the successful mission to "Challenger Deep" in the Mariana Trench under the western Pacific, was a technological breakthrough in marine engineering. Don Walsh describes the dive to Rebecca Kesby, and explains why understanding the deep ocean is crucial in the fight to reduce climate change.

(Photo: The Bathyscaphe Trieste in 1960. Getty Images)