Salt. It’s so common, we hardly think about it.

But salt is a vital ingredient—for the function of our nerves, muscles, and organs.

Humans used to get their salt from eating wild meat. But as we began to rely on agriculture, adding salt to our diets became more important.

For cultures that lived near the sea, getting salt was easy. But the farther inland humans lived, the more salt became a scarce commodity.

Wars were fought over access to it. And salt mines, when they were discovered, were strategic resources.

Salt reserves form when seas dry up and their salt is left behind as thick layers.

Salt is different than any other sediment layer: it flows like glaciers at Earth’s surface and forms giant salt domes underground.

As early as 3500 BC, a salt reserve was found in Poland. It became a center of commerce, and eventually the city of Kraków was built on top of it.

By AD 1200, miners had dug the mine more than 1,000 ft down, to include nearly 2,000 chambers. The miners began to build religious chapels in them, especially where there had been accidents.

The grandest chapel has furniture, statues, and even chandeliers, all carved from salt. The chapels are now a UNESCO World Heritage Site, so if you’re ever in Kraków, be sure to observe a quiet, if not salty, moment.