Ice XVI is the least dense (0.81 g/cm³)^[1] experimentally obtained crystalline form of ice. It is topologically equivalent to the empty structure of sII clathrate hydrates. It was first obtained in 2014 by removing gas molecules from a neon clathrate under vacuum at temperatures below 147 K.^[1] The resulting empty water frame, ice XVI, is thermodynamically unstable at the experimental conditions, yet it can be preserved at cryogenic temperatures. Above 145–147 K at positive pressures ice XVI transforms into the stacking-faulty ice I_c and further into ordinary ice I_h. Theoretical studies predict ice XVI to be thermodynamically stable at negative pressures (that is under tension).^[2][3]