A towering inscribed monument on a distant planet that is in fact an asteroid deflector, built by the Preservers to protect transplanted humans, in The Paradise Syndrome.
The obelisk stands near a village of people descended from several Native American nations, who were carried to the planet long ago by the mysterious race known as the Preservers. The villagers revere it as a temple but have lost the knowledge of how to use it.
In the Original Series episode The Paradise Syndrome, Kirk accidentally opens the structure when he speaks into his communicator, falls inside and is struck by a beam that wipes his memory. Emerging, he is hailed as a god, takes the name Kirok and marries the priestess Miramanee, while Spock and McCoy try and fail to turn aside an approaching asteroid with the Enterprise.
Spock eventually works out that the symbols on the obelisk represent musical notes. He reopens it, restores Kirk's memory and triggers the deflector beam, which pushes the asteroid away.