Western Pacific Ocean - He's plunging seven miles down a marine crevice named after Mariana of Austria/Queen of Spain's fathomless snatch.
A custom-built one-man sub called the DeepSeat Challenger sees the dive heading almost 'where no man has been before'.
Cameron is aiming to equal the successful 1960 voyage by ex-US Navy lieutenant Don Walsh and Swiss oceanographer Jean Luc - er...Jacques! - Piccard in the Bathyscaphe Trieste sub.
"These men share a fascination with the 17th century Queen," the head of US Navy Satire commented today, "it's driven these guys to probe some pretty murky alien depths."
Apparently the 17th century monarch married her uncle while aged about 11 and bore him five inbreeds - all either deformed or insane - before giving her name to the world's deepest crack.
Tonight Cameron has begun the plunge towards the Trench's deepest part called the Challenger Deep, described as 'a small slit-shaped valley' in the crevice floor.
A website covering the mission has commented that Cameron might have developed an obsession with Mariana and her wet, salty wench - er...Trench! - after viewing a 1652 oil on canvas portrait of her by Velázquez (now at The 'Prada' in Madrid).
It depicts Mariana in black velvet robes decorated with rows of thousands of entwined silvery fish.
Same as the DeepSeat Challenger's livery and Cameron's own wetsuit design.
He'll probably get stuck down there and never come back.