M-113 was an inhabited M-class planet. It was the homeworld of a type of poisonous plant known as the borgia plant, as well as the home of a civilization of M-113 creatures, which went extinct long before 2266.
The planet was hotter and more arid than Earth, so Human inhabitants needed to ingest larger amounts of salt to adjust for the loss of the mineral due to sweating. M-113 was once rich in salt but the planet's supply of the substance became depleted in antiquity, which was the cause of the eventual extinction of its native species of creatures, and by 2266, no salt could be found on the planet, so it had to be imported. Apart from the aforementioned borgia plant, M-113's landscape also featured dry brushes.
Several stone sculptures depicting various animals were found on the surface of the planet among vast ruins, including sculptures of a winged creature holding a sword, a small stone sculpture resembling a dolphin as depicted in Earth's Renaissance, a four-legged deer-like creature seated like a traditional Egyptian sphinx statue, and a sculpture of a lion-like animal.
In 2261, archaeologist Robert Crater and his wife Nancy began an expedition on M-113 to conduct a survey of the planet's many ruins. They moved into a large deserted ruin on the planet surface and began their archaeological research. During their first four years there, they shipped out large amounts of artifacts, gathered from several digging sites on the planet, made numerous reports, and were visited by a number of ships. A large drop in shipments was noted in their fifth year, however.
Unknown at the time of the Crater expedition, a single surviving M-113 creature continued to live on the planet. It eventually killed Nancy and continued to live along with Robert until 2266. Crater buried his wife on a hill on the planet, but was no longer as efficient in his archaeological work, causing the drop in shipments. (TOS: "The Man Trap")