The Warming
From Memory Alpha, the free Star Trek reference.
The Warming is a period of time in Ocampa history in which Ocampa's surface became a total desert. The Caretaker came and opened a chasm which allowed the Ocampa to flee underground.
The story of The Warming was told to Harry Kim and B'Elanna Torres by an Ocampa doctor. It was also told to Kathryn Janeway and Tuvok by the Caretaker himself. (VOY: "Caretaker")
