Sandstone is easier to carve than most other forms of rock, and that's why our ancestors preserved to work with it when making shelters whenever possible. The Longyou Caves, found in China, are particularly impressive. Some are one hundred feet tall, and they go on for 320,000 square feet.
Each cave is covered in symbols which cannot be translated. They don't match any form of language that we know of from 2200 years ago. Nor is the existence of the caves recorded in any known text. We're at a complete loss as to who carved them.