A sunken greenhouse is slightly buried within the ground during its construction, absorbing the ambient heat from the soil that its in contact with. This can be done by excavating either the soil found underneath the greenhouse, or partially embedding it within a hillside. The more contact with the soil, the more ability the building has to release or absorb the surrounding temperature.
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.This is an example of the flywheel effect, a law in thermodynamics. The flywheel effect means that portions of greater energy, or warmer masses, will naturally transfer heat to the cooler masses its in contact with.
