Best Heat Absorbing Material for an Underground Heat Sink

[JoeCal]

In building a subterranean heat sink for a greenhouse , what would be the best material to use in the heat sink through which the hot air of the greenhouse would be travel to warm. that is what materials would absorb the most heat easily and at the same time slowly release that captured heat?

Preferably, what would the a.) best material be b.) the best rock to use, c.) would it be better to be more solid (bricks, concrete) or a dense gravel mixture or a combination of both.

 

[BruceW]

The amount of energy stored per change in temperature is defined as heat capacity.
So I reckon you'd need a high heat capacity material. Water has a huge heat capacity, but I'm guessing you want to use a solid material. search heat capacity, wikipedia have a list of heat capacities for different materials.
You might want to ask someone who knows about this kind of stuff, there may be other important factors involved.