bathyscaphe

Edit: Apparently the DC weighs a tenth as much as the Trieste (the original bathyscaphe), including all the instruments.

The bathyscaphe Trieste was the first to reach the deepest point of the sea, back in 1960.

The linked document explains how the gasoline used for buoyancy is significantly compressible and becomes denser as the bathyscaphe descends.

What's terrifying on the bathyscaphe Trieste (pictured in the article) is that the 'living quarters' are only the small sphere attached underneath.

And other than the Costa Concordia, bathyscaphe liners are few and far between.

The Trieste bathyscaphe made it to the bottom of the ocean but we do not have underwater colonies, nor seasteading.

The bathyscaphes protect their bodies from instant disintegration, but they already have a prototype of a space-suit that will let them leave the bathyscaphe.

The bathyscaphe Trieste used 32000 gallons of gasoline to achieve positive buoyancy.

The 2014 World's Fair will have exhibits showing cities in the deep sea with bathyscaphe liners carrying men and supplies across and into the abyss.

Once in a while we send a bathyscaphe or two to the deep to explore the oceans bottom and the lifeforms there.

The bathyscaphe could be pressurised with a helium/oxygen/other gas.

Incidentally, I first came to know about the bathyscaphe while reading Peter Watts's Rifters trilogy (which is an amazing hard SF series set in the deep sea).

Otherwise you have a bathyscaphe, which has some other not nice failure modes (in addition to the "implodes if you f it up" one) and are much less maneuverable and arguably the whole system is less durable more costly for high tempo operations.

If you are interested, one of piccard books, Au Fond Des Mers en Bathyscaphe, explains everything from a personal perspective.

There has never been a human that deep, unless in a device called a 'Bathyscaphe', and even that is quite an undertaking, and definitely not without risk.