The basic physics of the underwater implosion of metal tubes is studied using small scale experiments and finite element simulations. A series of underwater implosion experiments have been conducted with thin-wall aluminum alloy 6061-T6 tubes. The nominal tube dimensions are 2.54 cm outside diameter and 30.48 cm length. Two cylinders collapsed at their natural buckling pressure of 6895 kPa gauge pressure (1000 psig). Two additional cylinders were caused to implode at 6205 kPa gauge pressure (900 psig) using an initiator mechanism. Each of the four cylinders failed with a mode 2 shape (collapsed shape is flat with two lobes). The near field pressure time-history in the water is measured at a radial distance of 10.16 cm (4in.) from the centerline at three points along the cylinder's length. The pressure time-histories show very similar behavior between the cylinders which buckled naturally and those which were mechanically initiated at 90% of the buckling pressure. To aid in understanding the physical implosion phenomena, a computational model is developed with a fluid-structure-interaction finite element code (DYSMAS). This model is validated against the experimental data, and it is used to explain the features of the implosion pressure pulse and how it is physically created.