FIGURE 1. Cyclic filling and flushing of a subglacial lake system.

a, Lakes U (upper) and L (lower) are isolated from each other and have differing hydraulic potentials (indicated by the water levels in the hypothetical manometers). Meltwater flows into lake U (purple arrow), raising its level and that of the ice floating on the lake (red arrow). b, The level of lake U and the overlying ice column continues to rise, increasing the hydraulic potential difference between lakes U and L (black arrows) until a drainage pathway that connects the lakes develops (star). c, Water transfer from lake U to L (blue arrow) causes the level of lake U to fall and that of L to rise. The motion of the ice columns overlying the two lakes can be monitored from space⁷, and can be likened to that of a pair of ice pistons moving through an ice cylinder. Ice deformation at the contact between the piston and cylinder resists this motion, possibly attenuating the flood.