Cicada nymphs create what we call turrets or chimneys above the tunnel hole where they will eventually emerge. These chimneys are made from soil. They are a positive indication of where cicada nymphs are underground and that they will emerge soon. Cicadas seem to build chimneys in wetter, muddier areas; in dry areas they will simply make holes at the surface. The chimneys will help keep water and mud from back-filling their holes, so they can continue to breathe, take a peek out and prepare to emerge.
Magicicada Tunnel (by Les Daniels):
A cicada tunnel in Bangkok Thailand (by Santisuk Vibul):
Three Magicicada chimneys (by Roy Troutman):