🕳️ Caves and underground

Most caves form when slightly acidic water slowly seeps through cracks in certain kinds of rock, like limestone, dissolving tiny bits of it away over an incredibly long time. Drop by drop, year after year, that slow dissolving carves out small cracks into larger tunnels, and eventually into huge open rooms underground. Other caves form in completely different ways, carved by flowing lava, crashing waves along a coastline, or even melting ice.

When water carrying tiny dissolved bits of mineral drips slowly from a cave ceiling, some of that mineral gets left behind each time a drop falls, building up ever so slightly with every single drip. Over thousands of years, those tiny leftover bits stack up into long, pointed shapes, stalactites hanging down from the ceiling and stalagmites growing up from the floor below them. Given enough time, a stalactite and a stalagmite can even grow so long that they meet in the middle and join into one solid column.

Rock is very slow to heat up or cool down, and deep inside a cave, surrounded by thick layers of rock on every side, temperatures barely change at all between day and night, or even between summer and winter. Most caves settle at roughly the average yearly temperature of the area above them, which is why a cave often feels cool in summer and surprisingly warm in winter compared to outside. This steady temperature is exactly why some animals choose to spend the coldest winter months resting deep inside a cave.

Cave explorers, called spelunkers or cavers, always carry multiple sources of light, since being lost in complete darkness underground can be extremely dangerous. They also carefully map each tunnel as they go, marking directions and distances, so both they and anyone who explores after them can find the way back out. Many cavers wear helmets with a light built right on top, which keeps both hands completely free for climbing and crawling through tight spaces.