but now john roebling was no more. the bridge's great towers, their gothic silhouettes recognizable all over the world are set on foundations deep beneath the east river. those foundations were sunk using case-ons, huge chambers set on the river's bed. inside these chambers, hundreds of men dug out sand and stone while blocks of granite and limestone built the great tower above. a case-on is launched like a ship from a dock, an upside down ship. it is towed to its correct position in the river and then sunk to settle on the mud bethe surface of the water. beneath the surface of the water. there were shafts to let in men and material and to bring waste material up as the men head down towards the bedrock far below. the case-on is made offed wood. its roof layer upon layer of dense pitch pine. there were shaft, too, to pump compressed air into the chamber. it's the compressedded air which keeps the river out. what's it like to work in come press air? it's like deep sea diving. come up too fast from the dense atmosphere,