Why can not submarines reach the deepest part of the ocean?

high water presure

Ans 2 - modern submarines dive deeper than ever before, yet still don't commonly go deeper than 1,500 feet or so. Submarine hulls are elongated cylinders and have many holes, all sealed by something, but nevertheless, weak points in the hull.

A submarine generally has at least 3 entry ports for personnel or weapons, ports for shooting torpedos or missiles, holes where planes and fins and periscopes are attached, and of course for the drive shafts. All of these holes prevent really deep diving. - Sealing against 1,500 feet of pressure is vastly different than sealing against 30,000 feet of pressure !

The deepest parts of the oceans are over 30,000 feet down with tremendous water pressure. The only devices to survive that deep are perfectly spherical and with the least possible number of hull penetration points.