According to the United Family Of Anomaly Hunters (yes, that is their actual name), there are walled cities on Mars. In a silent video, images captured by NASA’s Mars Reconnaissance Orbiter (MRO) in 2012 show what we are led to believe are alien walled cities on the Red Planet.
The structures are located in Elysium Planitia, an area of Mars with a fascinating geological history. It used to have a sea and volcanoes, and now it has mesas, valleys, and a deposit of ash-covered ice as big as the North Sea.
NASA’s official page has very little regarding these structures, and simply lists them under the very broad umbrella of volcanic processes. The full image is about 5 kilometers (3 miles) wide, making the structures around 100 meters (330 feet) across. They could have been formed by phreatomagmatic eruptions, which happen during explosive eruptions where water and magma are mixed.
The walls and domes have then been shaped by erosion, which has been slowly but surely eating away at them. And to the people questioning how such geometric shapes can be formed in nature, we'd suggest you check out the Giant's Causeway in Northern Ireland, another famous feature made by magma.
So there are no cities on Mars, but it would be funny if aliens built a wall before Trump.
Zoomed-in color image of the region. NASA/MRO