Something mentioned in an old army song.

https://www.youtube.com/watch?v=yJybwgtR970

a large watertight chamber, open at the bottom from which the water is kept out by air pressure and in which construction work may be carried out under water.

They use these when building bridges to keep the water out while building the piers. They have several purposes and uses.

I was thinking of the ammo box on a two-wheeled cart.  But I searched and see something about a water-tight chamber.  ? ?

I love Bob Crane's answer!!

Isn't that a note pad outside the front door.

In caisson not home, leave a mssge.

I sorta knew but better to rely on those who DO know. I liked Bob Crane 's answer!

Is it a type of wine?

No idea.

This is not a word that I would use in my descriptions.

Is it something that goes about the windows?       "A Window Caisson"

You've got me again. 

Does it have anything to do with air rights or something similar?

Anthony Acosta nailed this one!

Chinese water torture chamber.

Big poles/pipes/columns usually pile driven and anchored to "bed rock" or some type of "hard pan" that is usually integral to a solid foundation.

Tony and Suzanne Marriott, Associate Brokers Why would someone construct a home under water?

I ate one this morning!  Oops, thought you meant croissant! 

Not what I was thinking but mine was military!

I see others have mentioned it ... for underwater construction. 

Time for my thinking cap . . . 

Now there's a loaded question.   

Isn't this a repeat?  It is pillons into the ground for a steady foundation.

I know, same as in bridge building

think watertight!

a unit that allows underwater construction work to happen safely....