What is it?
This is an insidious form of erosion in some soils where channels and tunnels develop beneath the surface. It is insidious as it is not readily noticeable (apart from small sediment fans at the tunnel discharge point) until the surface itself is undermined.
More information is given at the tunnel erosion introduction page and the tunnel initiation and development page.
Tunnel erosion in a sloping agricultural landscape producing red-brown sediment fans at the outlets of the tunnels