Castle Frank Brook is a buried creek and south-west flowing tributary of the Don River in central and north-western Toronto, Ontario, originating near the intersection of Lawrence Avenue and Dufferin Street.

A subterranean river is a river or watercourse that runs wholly or partly beneath the ground, one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a permeable layer of rock or other unconsolidated materials. A river flowing below ground level in an open gorge is not classed as subterranean.