Sorry guys not use to this site or how it works, only just found it by accident.
Have the answers now, must be the first time have finished this xword!!!
Yeah its called a "caisson" they were used to construct foundations and pillars for such things as bridges. They were used to create the Brooklyn Bridge where men working in the Caisson suffered from "caissons disease" later known as The Benz, sorry if the spellings not right.