Complex sentence: The process is controversial, due to the highly toxic nature of cyanide; see also Summitville mine, the worst environmental mining disaster in United States history, which rendered 17 miles of a Colorado river devoid of life.
Split into simpler sentences: The process is controversial, due to the highly toxic nature of cyanide. The worst environmental mining disaster in United States history occurred at Summitville mine, rendering 17 miles of a Colorado river devoid of life.