[superhornet59]

actually i will be making an air bell possibly. i have a small creek and large basin, and im going to go to the government and see if i can make a dam to flood the basin, making a small lake/ large pond. there is a bridge with three "tunnels" that i will use as a base for my dam, and im hoping that one of the tunnels will go as such:

a wall goes from the top to one meter from the bottom, swimmer reaches bottom and does a U type thing (goes under wall then goes up). normaly it would flood to like 2 or 3 meters until pressure is equal, and either way water bottom will always be equal to how low the wall goes. instead there will be a "pool" to contain the water, and i will open a few scuba air tanks to get the pressure in there. itd be very very large (25 meters long by 3 meters high by 6 meters wide). would rock.

EDIT: and for those of you talking about gasses, there would be plants, special solar lamps would trigger photosynthesis, and negative ion air filters should get rid of other filth.