also: ぼたやま
noun
slag heap; coal waste pile
A mound of waste material from coal mining, often seen in former mining areas.
かつての炭鉱の近くには、今もボタ山が残っている。
Near the former coal mine, a slag heap still remains.
Compound of ボタ (slag, coal waste) and 山 (mountain). The term ボタ is of uncertain origin, possibly from a dialect word for waste or refuse.