Since the dawn of humankind, people have used caves—to explore, hold religious ceremonies, create art, or avoid the dangers of weather and predators. Partly because of that, they continue to fascinate scientists today.

To create a cave, Mother Nature needs three things: water, rock that can be dissolved by it, and lots of time.

Rainwater, as it falls through the atmosphere, picks up carbon from CO2 to become a weak carbonic acid. By the time it hits Earth, it’s about as acidic as coffee. As it percolates through the soil, it picks up more carbon from decaying plants, becoming a slightly stronger acid.

If the rock below the soil is limestone, gypsum, or dolomite, the water can dissolve along tiny cracks. Over many thousands of years, the cracks become channels, then tunnels, and could eventually become caverns.

Water might also mix with hydrogen sulfide gas seeping up from natural oil and gas deposits to form sulfuric acid, which can also dissolve the rock.

Protected from daily and seasonal changes on the surface, caves can maintain a stable temperature and humidity.

In these delicate environments, the remains of ancient animals and humans, which could have quickly decayed on the surface, have been preserved for millennia. Deeper, more isolated caves have preserved bacteria and microbes undisturbed for millions of years.

These qualities make caves important sites for researchers—natural time capsules. There’s probably an amazing cave near you, so take a trip and get to know your Earth.