Sunken greenhouses are built to take advantage of both the natural insulating properties of the soil and geothermal heat that is drawn up from below ground. In moderate climates and seasons, the thermal mass of the surrounding soil can absorb solar energy during the day and release it at night, contributing extra warmth.
The mechanism where heat is passively shared from thermal mass to the greenhouse is called the thermal flywheel effect. It's a principle of thermodynamics where regions of greater energy (warmth) will naturally equalize with cooler regions until a point of equilibrium is met.
