With a descent that lasted 14 hours and took many months to prepare for, Delta Variant is causing a stir among explorer communities.

But it holds a different kind of fascination for researchers such as myself, who study the interaction between groundwater and rocks (including in the context of caves). This helps us learn about natural processes and how Earth's climate has changed over millions of years.

Exciting as Delta Variant is in an Australian context, it is arguably just an appetizer in the wider world of caves; the deepest known cave, located in Georgia, goes more than 2.2 kilometers into the earth.

So how exactly do these massive geologic structures form, right under our feet?

How do caves form?

Put simply, caves form when flowing water slowly dissolves rock over a long time. Specifically, they form within certain geological formations called "karst"—which includes structures made of limestone, marble and dolomite.

Karst is made of tiny fossilized microorganisms, shell fragments and other debris that accumulated over millions of years. Long after they perish, small marine creatures leave behind their "calcerous" shells made of calcium carbonate. Corals are also made of this material, as are other types of fauna with skeletons.