Giant crystal caves are natural formations found deep underground, where large crystals grow over thousands of years. These caves often contain stunning mineral formations, such as selenite and gypsum, which can reach impressive sizes. The unique conditions in these caves, including temperature and humidity, allow crystals to develop in intricate shapes. One of the most famous examples is the Cueva de los Cristales in Mexico, known for its enormous selenite crystals that can be over 11 feet long. These caves are not only beautiful but also provide valuable insights into geological processes and the Earth's history.