The Onferno Caves are the only natural caves in the Conca Valley.
The caves' narrow tunnels were formed by a small stream that
over millions of years carved into the mass of chalk on which
Castrum Inferi once stood. Although they do not extend very
far, the caves are quite awesome and extremely interesting from
both a speleological and faunistic point of view. It is possible
to visit the caves. The Onferno Caves and the surrounding area
have been protected and an oriented nature reserve since early
1992.