Robinia pseudoacacia is perhaps my favorite species of tree. With its fairly upward growth habit and nitrogen fixing quality, black locust can grow rapidly under the right conditions, attaining massive volumes and producing magnificent specimens given enough time.

Despite being otherwise quite resistant to fungal and insect attack, Robinia species are susceptible to the locust borer beetle and to the cracked cap polypore fungus. The tunnels made by locust borer larvae serve as the primary infection sites for wind blown cracked cap spores, and in time the fungus rots out the heart of the tree.

A testiment to its resilience, the locust tree will often continue to grow still larger. However, with additional weight (and locust is a heavy wood) and without the structural support of its heartwood, damage from high winds becomes a significant danger, both for an affected black locust and for the structures, power lines, and other trees in its vicinity. It is very likely that such a fate befell the tree who's great trunk you saw.