Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved February 12, 2021 (received for review December 8, 2020)Under typical conditions on Earth, ice exhibits two distinct polytypes: hexagonal and cubic. Yet, despite its importance to nature (e.g., the atmosphere) and technology (e.g., cryopreservation), the crystallization of liquid water to cubic ice has never been achieved in the laboratory. Here, we show how to design substrates to nucleate desired polytypes of ice, including the elusive cubic ice, and show that the process is controlled by the structure of...…Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved February 12, 2021 (received for review December 8, 2020)Under typical conditions on Earth, ice exhibits two distinct polytypes: hexagonal and cubic. Yet, despite its importance to nature (e.g., the atmosphere) and technology (e.g., cryopreservation), the crystallization of liquid water to cubic ice has never been achieved in the laboratory. Here, we show how to design substrates to nucleate desired polytypes of ice, including the elusive cubic ice, and show that the process is controlled by the structure of...WW…