hotbed

Short definition from Wiktionary

Noun

1. 1
   A low bed of earth covered with glass, and heated with rotting manure, used for the germination of seeds and the growth of tender plants, like a miniature hothouse.
2. 2
   An environment that is ideal for the growth or development of something, especially of something undesirable.
3. 3
   An iron platform in a rolling mill, on which hot bars, rails, etc., are laid to cool.