The world famous Elbe River runs directly through the picturesque Czech Republic city of Děčín, a major avenue of activity since ancient Roman times and even earlier. It makes its way there via Prague, then curls into Germany and winds through cities like Dresden and Hamburg before emptying into the ocean, just south of Denmark. It's a major waterway for much of central and eastern Europe. And when it starts to dry up, bad things tend to happen.

The Hunger Rock of Děčín wasn't unknown before, but every time it makes an appearance, it does so with grim theatricality. "Wenn du mich siehst, dann weine," its German text reads: "When you see me, weep." If the stone appears, it means that the water has dried up, that the fields will go dry, that times of hunger are upon us. It also means that transportation on the river will be a lot harder than usual, or even impossible. Of course, the 2018 appearance of the stone wasn't the first time it surfaced. In fact, besides the dire warning, the stone is inscribed with every year that it made an appearance up until 1893. The first year recorded is 1417, although the stone wasn't actually carved until the second date on its surface, 1616.

This isn't the only ancient warning, however. With the Elbe at its lowest in 50 years, you can find many other hunger stones emerging ominously from the water. In Germany, the water levels are revealing warnings like "If you will again see this stone, so you will weep, so shallow the water was in the year 1417," "We cried – We cry – And you will cry," and "Who once saw me, he cried. Whoever sees me now will cry." It's not all doom and gloom, however. Back in Děčín, a Czech-speaking prankster made one impish addition to the dire message sometime in the past four centuries: "Don't cry, girl, don't fret. When it's dry, just spray your field wet."

On a happier note for historically minded scholars, the heat wave that wreaked havoc on Europe this past summer has revealed a lot more than scary messages on an ancient stone. As vegetation grows on the parched landscape, even slight variations in the soil can become dramatically highlighted. The result? Across the British Isles, dry conditions are showing the locations of ancient structures never before marked in history books. These ancient sites have begun to stand out on the landscape, dramatically highlighted in yellow or green.

Similar conditions in the past have revealed these ancient forts and henges as well, but thanks to the advent of drone technology (and the especially dry summer), 2018 has been a banner year for these kinds of discoveries. In Wales, round-cornered squares indicate prehistoric or Roman farms, and in Ireland, a prehistoric henge about 5,000 years old has been revealed in breathtaking precision. What determines if they turn green or yellow? Basically, it's the quality of the soil. A moat, for example, filled in with fresh soil after the fortress has crumbled away, will often be moister and more fertile than its surroundings. By contrast, any site where stones still lie beneath the surface under thinner topsoil will dry out faster than the surrounding dirt. Either way, the contrast indicates historic roots.

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Some ancient monuments don't need a drought to reveal them. "Hadrian's Wall" by Adrian Goldsworthy tells the story of the edge of Roman civilization and the beginning of Ireland. It's free if you're trying out Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.