[snarf]

16,500 PSI was just what it was tested at, and it could apparently withstand significantly greater pressure than that. From the Deep Sea Challenge website:

"The hull, complete with its hatch and viewport, was tested twice in a pressure chamber at Pennsylvania State University to an equivalent full-ocean-depth pressure of 16,500 pounds per square inch (1,138 bars). It passed both tests. Twenty-two strain gauges attached to the sphere gave data that indicated the sphere could withstand up to 140 percent of the test pressure without buckling."