Acid

An acid is a type of chemical compound. Strong acids can react violently, with a corrosive effect on metals.

The Horta have glands which produce acid which allow them to burrow through rock. When a Horta is confused or scared they sometimes loose control of their acid glands and squirt their acid around them, which can cause extensive damage to anything it happens to come into contract with. (TOS episode: The devil in the Dark; DS9 novel: Devil in the Sky)