Romania has more than its share of creepy things that seem to be alive but aren’t. Take, for instance, the “living” rocks that grow and move. These Romanian curiosities are called trovants and can sometimes resemble gigantic fossilized dinosaur turds. They’ve been around far longer than old Vlad Dracula, but they don’t bite and you can visit them in daylight. Less creepy than mythical vampires, sure, but these things actually exist.

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Being who I am (and being as old as I am), when I first read about trovants my thoughts zoomed straight to some classic sci-fi creatures. Who remembers the Horta from the Star Trek TOS episode “Devil in the Dark,” that poor misunderstood mother who was only trying to protect her perfectly round egg nodules? Thank goodness Spock realized he needed to mind-meld with her.

In another Star Trek TOS episode entitled “The Savage Curtain,” Earth’s Abraham Lincoln and Vulcan’s Surak are conjured up by a gravelly-voiced pile of boulders trying to understand the concepts of good and evil. The rocky alien also produced some bad guys, including Genghis Khan, and initiated a fight to the death between factions. This episode is right up there with “Spock’s Brain” in many ways.

But the best one is, hands-down, the rock monster from Galaxy Quest. It consisted of scattered rocks and boulders that it could pull together at will. Captain Peter Quincy Taggert (Tim Allen) loses his shirt while dodging and diving to get away from it in one of the best sci-fi comedies spoofs of all time.

Trovants aren’t that mobile and are (most likely? as far as we know?) not sentient. They are rocks, geologic formations. How does a rock grow much less move? Some theories advanced over the years have speculated that trovants are alien artifacts, or that they are responding to and interacting with weird magnetism, or that there are unexplained energy vortexes in the local area.

Trovanti Museum Natural Reserve is located in southern Romania among the sand quarries of the Vâlcea district near the village of Costeşti. The trovants there range from a few grams to several tons, with the largest reaching a height of 10 meters.

Consisting of cemented sand and mineral salts, when a trovant is cut in half you can see “age rings” like you find when you cut down a tree. These rocks started out as a hard stone core with a shell formed around it made of sand. Trovants can only form in areas with highly porous sand accumulations and sandstone deposits that are cemented by waters rich in calcium carbonate. Locals have always known, and geologists have confirmed, that trovants grow after heavy rains when they absorb the rain’s minerals. The minerals combine with chemicals already present in the stone that later creates a reaction and pressure inside. The pressure makes the rock grow from the center outward and sometimes produces lumps and bumps. This process takes a very long time, and it only occurs in areas with a type of porous sand consisting of a certain chemical composition that receives the right amount of rainfall carrying the right mineral mix. It’s quite a complex recipe.

The chemical processes at work on the stones cause them to bulge and form lumpy protrusions. When a bulge grows large enough, the force of gravity may cause it to drop off the parent stone. The baby trovant continues to grow, absorbing minerals through rain water, potentially producing its own lumps that will one day separate from it—and the cycle continues.

These things grow, and move, and reproduce like eerie rock monsters that should only exist in science fiction. What else do geologists know that they aren’t telling us?