A grandson of French oceanographer Jacques Cousteau is following in his legendary footsteps, heading to the deep for a record 31 days.
"It is symbolic for me personally, because my grandfather [and team] spent 30 days underwater in the Red Sea 50 years ago," said Fabien Cousteau, 46, in the Florida Keys.
He is taking part in experiments and work with documentary filmmakers in an undersea lab called Aquarius off Key Largo, south of Miami.
It features internet, air conditioning and portholes to take in the views, plus higher-than-normal atmospheric pressure.
If he makes it to his target, Cousteau - who started diving at the age of four - said he would break his grandfather's record to "honour" his legacy and "his aquanauts and all the previous aquanauts who have done such fantastic work".
He and his team of researchers, engineers and cameramen headed down to the 20-metre Aquarius yesterday.
Three of the researchers, led by Cousteau, will stay put under water until July 2, observing sea life, the effects of pollution on coral and of long-term underwater stays on people.
They will dive for about 12 hours a day.
Cousteau said his grandfather, who died in 1997, used to say that "in order to film a fish, you have to become a fish".
"And what better way to film the unknown and the final frontier on our planet, than to actually become a fish for 31 days," he said. "This is a very complicated and difficult endeavour, it's a very unique project."
Life in Aquarius "is very difficult. It is very humid inside. We have an air conditioner on all the time just to try to keep the living quarters semi-dry".
"We will be getting regular deliveries of clothes, so every four or five days we will get new T-shirts from the surface."
Readjusting to normal atmospheric pressure and gradually moving up towards the surface takes a staggering 18 hours.
Those who are taking part have been training for weeks, including contingency plans in case of an emergency.