Ferric oxide

Ferric oxide (Fe2O3), commonly known as rust, was a compound made up of high levels of corroded iron particles.

In 2371, the crew of the USS Voyager detected this compound in open space while traversing the Delta Quadrant. Its detection raised the question, from Captain Janeway, "how could iron rust in space without oxygen?"

Unable to determine an answer, they decided to follow the "trail of rust" to determine its source, which led them to the discovery of complex hydrocarbons in the rust, notably benzene, ethylene, and acetylene, the components of gasoline. Once located, the source was discovered to be a 1936 Ford truck, formerly belonging to an Ohio farmer. 

When presented with the idea of traveling back to the 20th century aboard the HMS Bounty, Leonard McCoy asked James T. Kirk if he was "really going to try this time travel in this rust bucket?"