subterranean rivers (AB ev)

underground river inside the Yellow Dragon Cave

The River Styx is an underground river flowing through Mammoth Cave that exits the cave system and flows into the Green River.
See www.recreation.gov

Clear Water Cave

Jenolan River / Styx River
See en.wikipedia.org

Medicine Lake, which drains via an underground river into the spectacular Maligne Canyon (AB ev)

Domica Cave