What is a cave

A cave is just space, where the rock used to be.
Until water dissolved it away, to the sea.
While the water erodes it, and eats it away.
The cave just gets bigger, and bigger, each day.
How “can” this be so, I can hear people say,
How can something get “bigger” as it’s eaten away.
I know it seems odd, and a little unfair.
But a cave, is a cave, just because it’s “not there”.