  Supersolidity -- a quantum-mechanical phenomenon characterized by the
presence of both superfluidity and crystalline order -- was initially
envisioned in the context of bulk solid helium, as a possible answer to the
question of whether a solid could have superfluid properties. While
supersolidity has not been observed in solid helium (despite much effort),
ultracold atomic gases have provided a fundamentally new approach, recently
enabling the observation and study of supersolids with dipolar atoms. However,
unlike the proposed phenomena in helium, these gaseous systems have so far only
shown supersolidity along a single direction. By crossing a structural phase
transition similar to those occurring in ionic chains, quantum wires, and
theoretically in chains of individual dipolar particles, we demonstrate the
extension of supersolid properties into two dimensions, providing an important
step closer to the bulk situation envisioned in helium. This opens the
possibility of studying rich excitation properties, including vortex formation,
as well as ground-state phases with varied geometrical structure in a highly
flexible and controllable system.
