Mars is crossed by a planet-wide network of long, straight canals visible from Earth.

During the favorable Mars opposition of 1877, Giovanni Schiaparelli mapped narrow features he called canali—Italian for channels. In English, “canals” carried a stronger suggestion of artificial construction, and later observers, most famously Percival Lowell, drew elaborate networks of straight lines across the planet.

The claim was never unanimous. Some skilled astronomers could not reproduce the features at all, and increasingly powerful telescopes failed to make the supposed geometry more secure.

The problem lay near the limits of visual resolution. Faint, disconnected patches and boundaries can be perceptually joined into straight lines when viewed through a small, turbulent telescopic image. The resulting maps could become more orderly than the surface that produced the impression.

Mariner spacecraft finally photographed Mars at close range. The geometric network was absent. Later missions revealed a far richer real landscape of impact craters, volcanoes, dunes, fractures, valleys, and channels carved by ancient water and other processes.

Those genuine channels should not be confused with Lowell’s planet-spanning straight canals. The historical claim is best understood as a prominent and contested telescopic interpretation, not a unanimous astronomical consensus.