Syani or Syanskie stone mines is a Moscow artificial cave system, where limestone was excavated for the city building. But nobody knows the exact date the development work beginning. The total length of the cave system is about 18850 meters,
which makes Syani the largest stone mine in the Moscow region and one of the largest stone mines in the whole country. Moreover this figure is not exact - some passages and manholes are choked and are still being explored.