Some patterns, when seen in nature, raise suspicion. Case in point: check out these bizarre patchwork cloud lines (shown above) that seem to etch out a grid across the ocean off the coast of Portugal and Spain. If you think this pattern looks unnatural, you're right.

Captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, the image represents the telltale signs of so-called "ship tracks." (Sorry conspiracy theorists, these aren't UFO trails.) They are known to form when water vapor condenses around tiny particles emitted by the exhaust of ships sailing across open ocean, reports NASA. They are low-lying formations, and are typical when stratus and cumulus clouds are present.

Because they form from the pollution emitted by ships, they trace out the routes that those ships travel. It's an alarming reminder of just how dramatic the impact of human industry can be on large-scale weather formation.

Ship tracks appear particularly prominent in part because clouds infused with ship exhaust have more and smaller droplets than unpolluted clouds. This causes light that ricochets off the clouds to scatter more, thus making the tracks appear peculiarly bright and thick.

These geometric trails can stretch for hundreds of kilometers before eventually dissipating. Like with contrails from jet engines, the narrow ends of the clouds are youngest, and the broader ends are older. In this way you can also decipher the direction that the ships are traveling in, as if the clouds form a giant arrow in the sky.