The Naica crystal cave :
The Naica Mine of the Mexican state of Chihuahua is a working mine that is known for its extraordinary selenite crystals. Some of the selenite crystals are as large as 4 feet in diameter and 36 feet long, making them the largest crystals ever found. The selenite crystals were formed by hydrothermal fluids emanating from the magma chambers below. The cavern was discovered while the miners were drilling through the Naica fault, which they were worried would flood the mine.
Conditions in the caves are incredibly dangerous - 50 C heat and near 100% humidity means that water from the air rapidly condenses in a person's lungs. Simply standing in the cave breathing will condense enough water that you begin to drown within ten minutes. Staying any longer means wearing a special suit with a cooling system and breathing specially dried air from breathing gear.
To the mining company, the cave is an oddity with no real value. The crystals themselves have no intrinsic worth, and there is no money to be made from tourism given how dangerous and inaccessible the caves are. Eventually when the mine becomes unprofitable the caves will be abandoned and allowed to re-flood, at which point they will be lost to us forever.
The startling thing is that many more and larger surprises like this are probably waiting to be found in the Earth's crust. Only the most miniscule fraction of the crust has been explored throughout history - we know far more about conditions at the edge of the solar system than we do about conditions even one mile under our own feet.
Give a man a fire, and you keep him warm for a day. SET a man on fire, and you will keep him warm for the rest of his life...