They were filled with compressed air, and often when the workers emerged after an extended period of working under this artificial pressure they experienced mild symptoms like tingling or itchy skin.
This meant that the work had to stop and the caissons had to be fixed in order to continue the work, and so, like most construction projects, it took much longer than they had planned.
Meanwhile, pre-construction dredging gets underway at the site location, before another ship lays the foundations for the caissons to lie on around the perimeter of the island.