"Root canal" is the term used to describe the natural cavity within the center of the tooth.

The prison was deep, natural cavity far beneath the surface of a secret planet and hidden forever from the stars.

To the human, the natural cavity seemed like the weightless center of a hollow asteroid, but it could be more complicated than that.

Nests are often (but not always) built in natural or artificial cavities.

It usually uses natural cavities but often takes over an abandoned woodpecker nest.

All the species use natural or disused cavities for nest sites.

There was a cramped natural cavity behind the main buttress roots.

Was this a natural cavity or a chamber hewn by man?

This is in keeping with the way bees build wax in a natural cavity.

This rock debris had filled some natural cavities and hallways.