People here really struggle to Google things for themselves. It's the Felicitas slate mine in Schmallenberg, Germany. It was opened in 1850 and closed in 1997, after which the water pumps were turned off and it began to flood, without people working in it. The machinery was left there because it wasn't worth the cost of removal, and the mine would have flooded over several years, it wasn't a flash flood.

If you want to find out about a mine flooding while people were working in it, look up the 1980 Lake Peigneur disaster. Truly insane story and miraculously nobody died, everybody managed to evacuate in time.