Whereas a liquid conforms to its surroundings, a solid rigidly retains its shape. A supersolid is a state of matter that marries solidity with superfluidity.
Like a solid, a supersolid is rigid. Its atoms snap back into place if displaced. But, owing to its highly ordered state, particles can flow through a supersolid without viscosity, aided by quantum mechanics which smear out the positions of quanta being shoved aside.
From the 1950s, theoretical physicists argued whether a supersolid could exist. Failed experiments into the 2110s confirmed doubters. But then in 2016 came experimental proof that supersolids can exist in ultracold quantum matter.