We have quantum mechanically calculated the 70 diffusion barriers of ethane moving through an HCN crystal to be ∼17 kJ/mol (Fig. ). Our estimate is likely an upper bound to the real barrier because it derives from calculations on a limited unit cell that exhibits substantial lattice displacement during 75 ethane diffusion. Nevertheless, the predicted barrier height permits cross-lattice diffusion of ethane on millisecond timescales near the 90-94 K surface conditions of Titan (33). Methane, a predicted major component of Titan seas, is assumed to have We can speculate that in the dynamic lake regions, seasonal variations might promote co-crystal formation during low tides, with hydrocarbons seeping out during high tides, leaving cavities behind. These processes could result in less dense, 'Swiss cheese'-like phases of HCN, or products derived thereof, offering another model to explain the enigmatic 'magic islands' observed in Titan's lakes . Our calculated co-crystal densities and estimates of the corresponding ethane-evacuated structures are shown in Figure , alongside an approximation to Titan's lake density (∼0.64 g/cm 3 ). Buoyancy is predicted only for partially evacuated co-crystals that have an original mole fraction of ethane exceeding 0.3.