Earth from space
Human activity has led to a few large simple geometrical structures that are visually noticeable from space. One is the almost-straight 30-mile railroad causeway built in 1959 that divides halves of the Great Salt Lake in Utah where the water is colored blue and orange. Another is the almost-circular 12-mile-diameter national park created in 1900 that encloses ungrazed vegetation on the Egmont Volcano in New Zealand. On the scale of a few miles, there is also rectilinear arrangement of fields in the U.S. Midwest, as well as straight-line political boundaries with different agriculture on each side. Large geometrical patterns of logging were for example briefly visible after snow in 1961 near Cochrane, Canada—as captured by an early weather satellite. Perfectly straight sections of roads (such as the 90-mile Balladonia–Caiguna road in Australia), as well as the 4-mile-diameter perfectly circular Fermilab accelerator ring are not so easy to see. The Great Wall of China from 200 BC follows local topography and so is not straight.
Some of the most dramatic geometrical structures—such as the dendritic fossil drainage pattern in south Yemen or the bilaterally symmetric coral reefs around islands like Bora Bora—are not artifacts. The same is true of fields of parallel sand dunes, as well as of almost-circular structures such as the 40-mile-diameter impact crater in Manicouagan, Canada (highlighted by an annular lake) and the 30-mile-diameter Richat structure in the Sahara desert of Mauritania. On the Moon, the 50-mile-diameter crater Tycho is also almost circular—and has 1000-mile almost-straight rays coming out from it.
At night, lights of cities are obvious—notably hugging the coast of the Mediterranean—as are fire plumes from oil rigs. In addition, in some areas, sodium streetlamps make the light almost monochromatic. But it would seem difficult to be sure that these were artifacts without more information. In western Kansas there is however a 200-mile square region with light produced by a strikingly regular grid of towns—many at the centers of square counties laid out around 1870 in connection with land grants for railroad development. In addition, there is an isolated 1000-mile straight railroad built in the late 1800s across Kazakhstan between Aktyubinsk and Tashkent, with many towns visible at night along it. There are also 500-mile straight railroads built around the same time between Makat and Nukus, and Yaroslavl and Archangel. All these railroads go through flat empty terrain that previously had only a few nomadic inhabitants—and no settlements to define a route. But in many ways such geometrical forms seem vastly simpler to imagine producing than for example the elaborate pattern of successive lightning strikes visible especially in the tropics from space.