  Although it is commonly expected that a metal disk placed on the surface of
water will sink, our investigation has revealed a surprising phenomenon: a
vertical jet directed onto the disk from above can allow it to remain afloat.
This result defies intuition, as one would assume that the force of the jet's
impact would cause the disk to sink.We have discovered that this phenomenon
occurs as a result of water displacement from the top of the disk caused by the
impacting jet, operating through a mechanism similar to a classical hydraulic
jump. This displacement generates a difference in hydrostatic pressures,
resulting in an upward buoyancy force capable of balancing the force of
gravity. In contrast to the classical case, here the jump radius is fixed by
the geometric parameters of a disk, a phenomenon we refer to as the parametric
hydraulic jump. To further explore this effect, we have presented a theoretical
model based on scaling laws, which provides the conditions required for the
disk to float. The dimensionless scaling constant's value was determined
through an independent experiment. Finally, we conducted experiments on the
disk's floating and sinking, which showed a good match with the proposed
theory.
