An ice spike!!! How does this happen? According to the Nonlinear Physics Group at the University of Toronto:
[A]s the ice freezes fast under supercooled conditions, the surface can get covered except for a small hole. Water expands when it freezes. As freezing continues, the expanding ice under the surface forces the remaining water up through the hole and it freezes around the edge forming a hollow spike. Eventually, the whole thing freezes and the spike is left.Neat! Hooray for science!