It was a record dive depth set by United States Navy Lieutenant Don Walsh and the late Jacques Piccard on January 23, 1960, and has never been beaten.
Now, a Dunedin man, who has met Piccard and worked with Walsh, will mark the 50th anniversary of the feat by giving a lecture about the expedition.
Marine biologist and writer Peter Batson (34) said the duo used the bathyscaph Trieste (a "bizzare-shaped" submersible) to dive to the bottom of the Marianas Trench, 200 miles off the Pacific island of Guam.
The journey was not without drama.
At a depth of about 10km below the surface, the water pressure was so great a window cracked in the bathyscaph's entrance tunnel, he said.
Half a century later, no-one has returned and no manned submersible operating today can dive that deep.
For Mr Batson, the pair were his childhood heroes.
"I knew who these guys were when I was a kid, because I was an ocean nut, and they were very famous.
"What they did was the equivalent to what astronauts have done."
Mr Batson said he met Piccard shortly before his death in 2008, and got to know Walsh while working with him on several deep-sea expeditions.
"I first met Don on an expedition to the Titanic in 2005.
"He is a huge man.
"It's hard to believe he was able to fit inside the [bathyscaph Trieste] sphere. There wasn't much room in there."
Mr Batson's hour-long talk at 4pm today at the New Zealand Marine Studies Centre, Portobello, will celebrate the bathyscaph's achievement, the courage of Jacques Piccard and Lt Walsh, and what they saw as they descended into the crushing depths of the bottom of the world.
