The Jacksons received help from sub experts on the Internet, many of whom urged them to install a drop-weight system for emergencies. They ended up cutting or casting 3,500 pounds of lead blocks that fit jigsaw-like into the exposed steel frame on the bottom. If they need to ascend immediately, they can turn a handle that releases all the blocks at once. With 3,500 pounds shed, the sub, weighing a scant 1.25 tons, rockets up to the surface. The sudden change in pressure could cause the sub to explode, though, so Doug says that in all but the direst circumstances, "I'd swim out and leave the sub on the bottom."