hotbed; breeding ground
Used both literally (e.g., a heated seedbed in agriculture) and figuratively for a place or environment where something, often negative, develops rapidly.
この地域は犯罪の温床になっている。
This area has become a hotbed of crime.
温床で苗を育てる。
Grow seedlings in a hotbed.
This area has become a breeding ground for crime.
The opaque system provides a breeding ground for fraud.
I made a hotbed to grow the seedlings.