Giant spiders crawl around the walls of an ancient city on Mars. And if that doesn’t sound like the plot of a bad movie, then what does? It turns out, though, that this plot is true – in a way. The “city” consists of walls of volcanic rock or hardened sand. And the “spiders” are explosions of dark dust from below the surface.

The region is called Inca City. It was discovered in 1972, in pictures from a Mars orbiter. It’s a grid of intersecting lines that resembles the outline of an ancient Incan city. The walls are miles long and hundreds of feet tall.

Scientists aren’t sure what created the lines. They could be hardened sand dunes. The most recent idea says they’re volcanic rock. Inca City may lie inside an old impact crater. Molten rock could have bubbled up through cracks in the crater floor. The floor was covered up, but the Martian winds have swept it clean, exposing the ridges.

Inca City is near the edge of the southern polar ice cap. Frozen carbon dioxide covers the region in winter. In spring, it vaporizes. Carbon dioxide a few feet below the surface can vaporize first, blowing holes in the ice above it. Dark dust in the plumes then settles to the surface – forming “spiders” around the walls of Inca City.

Look for Mars before and during dawn now. It looks like a bright orange star. Tomorrow, it’s to the upper right of the crescent Moon. The brilliant planet Jupiter stands above them.

Script by Damond Benningfield