Wave-depression cavities generated in the breaking regime of axisymmetric mode in a vertically oscillating circular cylinder is investigated experimentally. High-speed images are acquired and processed to study cavity behaviour during the implosion. The cavity collapse and subsequent jet formation are investigated, and jet velocity scaling has been proposed based on experiments. In deep water cavity collapse, the radial length of the cavity follows a power law r∼τ^α, where α is found to be ½ in the initial stage of collapse. A viscous transition occurs when the viscosity of the fluid is larger. An impulse model is derived, which gives some physical insight about the cavity implosion and the radial and axial gradients of the impulse. A high-velocity jet is obtained when the viscous dissipation dampens the disturbance wavelengths on the cavity interface. During cavity collapse with certain viscosity of the fluid, an inertial-viscous transition leads to a cusp singularity such that ((z-Z_0))⁄R∝ (r⁄R)^(2/3), which eventually leads to an extreme jet velocity of 120 m/s or higher. Further, the effect of the depth of liquid is also investigated. At shallow depth, the cavity collapse shows a viscous-inertial transition which is an indication of strong dissipation from the bottom wall.