Hotbed

A hotbed is a biological term for an area of decaying organic matter that is warmer than its surroundings.

The heat gradient is generated by the decomposition of organic substituent within the pile by microorganism metabolization.

[1] (The digestive systems of ruminants such as cattle and sheep destroy and use all cellulose in their food, and their droppings remain cold and do not heat up.)

[2] Data from this study does suggest that the amount of heat released by hotbeds does decrease after some time, however.

By extension, the term hotbed is used metaphorically to describe an environment that is ideal for the growth or development of something, especially of something undesirable.