The water below the ice remains liquid since geothermal heating balances the heat loss at the ice surface. The pressure causes the melting point of water to be below 0°C. The ceiling of the subglacial lake will be at the level where the pressure melting point of water intersects the temperature gradient. In Lake Vostok the ice over the lake is thus much thicker than the ice sheet around it.
There is also evidence that there are subglacial lakes on Jupiter's moon Europa. Not all lakes with perennial ice cover can be called sub-glacial, though, since there are also those that are covered by regular lake ice. A criterion for glacial ice is that it is flowing. Ice needs to be approximately thirty metres thick to start flowing, so frozen-over lakes are unlikely ever to transform themselves into subglacial lakes.