The deep sea is much harder to explore than outer space. Far from the sun’s warming rays, the inky darkness miles down hovers at temperatures near freezing. Seawater is also corrosive, often full of debris and largely opaque to light and radio waves.

Most daunting of all, it is extraordinarily heavy. In the Challenger Deep, the waters overhead exert a downward pressure of more than eight tons per square inch.

People have descended to the Challenger Deep just once before, in 1960, when the United States Navy sent down two men. They stayed 20 minutes. (By contrast, a dozen men have walked on the moon and spent more than a collective 80 hours exploring its surface.)

— William Broad writing for the New York Times