Assertion: mammoth cave | /r/HasProperty | world's known cave system
Facets:
Source sentences:
text: Mammoth Cave is the world's longest known cave system with more than 400 miles (640 km) of surveyed passageways, which is nearly twice as long as the second-longest cave system, Mexico's Sac Actun underwater cave.
text: Mammoth Cave, Kentucky, is a well-known example of a cave formed by this process.