Miners on Janus VI were being killed by a mysterious creature, which Spock learned was a Horta mother protecting the eggs the miners had been destroying.
The pergium mining colony on Janus VI suffered a series of deaths in 2267, with miners and equipment dissolved as if by acid. The Enterprise was called in, and Kirk and Spock hunted the creature in the tunnels. They found it was the Horta, a silicon-based being that moved through rock, and Kirk wounded it with a phaser.
Spock mind-melded with the Horta and learned that it was the last of its kind, guarding thousands of eggs. The miners had been breaking the eggs, which looked like silicon nodules. McCoy treated the wound with thermoconcrete, and the two sides agreed to work together, with the young Horta tunnelling for minerals.