Barry Smith maintains a growing site on "ontology" in its different aspects. Apart from "Formal Ontology", "Biomedical Ontology", "Cognitive Ontology" or "Social Ontology" it contains an interesting section on Geospatial Ontology.

The links assembled here refer e.g. to a project instigated by Barry Smith and David M. Mark that focuses on the ontological status of geographic categories.

If you ever wondered whether mountains actually exist, go there and find out. ;-)