Mateusz Konieczny
leisure=nature_reserve includes protecting geological structures - search for word geological

https://wiki.openstreetmap.org/wiki/Tag:leisure%3Dnature_reserve

"A nature reserve is a protected area of importance for wildlife, flora, fauna or features of **geological** or other special interest, which is reserved and managed for conservation and to provide special opportunities for study or research."

This object has wikipedia link leading to - this site contains some kind of rare salt crystals so I believe that tagging is correct