Sentinel

A sentinel is an alien artifact that was first discovered on Earth's moon, standing twice the height of a man, but pyramid shaped. It was featured in Arthur C. Clarke’s 1951 short story "The Sentinel", and was later adapted as a rectangular monolith in 2001: A Space Odyssey and its accompanying film.

The Sentinel
A sentinel is an alien artifact made of a polished mineral, is pyramidal in shape, and is surrounded by a spherical forcefield. The structure is found on Earth's moon, but was left behind eons ago by ancient aliens. It is located in a lunar mare, presently called Mare Crisium. The object is one of millions that are scattered throughout the known universe. The structure may be using "technology that lies beyond our horizons, perhaps to the technology of para-physical forces." Evidenced by dust buildup, the artifact may have been transmitting signals into deep space for millions of years, but it ceases to transmit after it is destroyed "with the savage might of atomic power". The narrator suggests that this "sentinel" was left on the moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth.

2001: Space Odyssey
The concept of the sentinel, featured in the 1951 short story, appears in the 2001: A Space Odyssey and in the film adaptation as the TMA-1 monolith. The operation of the monolith is activated when sunlight touches it for the first time after it is dug up. The shape is changed to a standing rectangle, and its location is set in the Tycho lunar crater. Arthur C. Clarke considered the TMA-1 monolith as the final product, and likens the Sentinel to be as just a seed.