The stability of so-called two-dimensional (2D) ice is re-evaluated. Recent experiments on ice formed by water under nanoconfinement provide evidence for a two-dimensional (2D) “square ice” phase. However, the interpretation of the experiments has been questioned and the stability of square ice has become a matter of debate. In this paper we have carried out diffusion Monte Carlo (DMC), DFT and force field calculations. We find that at relatively high pressure, square ice is indeed the lowest enthalpy phase examined, supporting the initial experimental claim. The work was mainly driven by my colleagues in London and published in Phys. Rev. B as rapid communication.