Which cave system has the greatest surveyed passage length?

Which cave system has the greatest surveyed passage length? Mammoth Cave does, in Kentucky, United States.

Mammoth Cave is a vast network of limestone passages formed as groundwater dissolved the bedrock beneath the Pennyroyal Plateau. Surveyors have mapped hundreds of miles of connected passages, and the measured total continues to change as exploration finds and documents new sections. Recent listings put the system at roughly 686 kilometres of surveyed passage.

Its name can be misleading: Mammoth refers to the cave's immense size, not to prehistoric mammoths living underground. The cave also is not simply one straight tunnel; it is a complex maze of chambers, shafts, rivers and crawling passages developed over geological time.

Sistema Ox Bel Ha in Mexico is the leading underwater rival, while Jewel Cave and Wind Cave are also exceptionally long systems. They do not surpass Mammoth Cave's total surveyed length. Mammoth Cave National Park protects the principal system and was designated a UNESCO World Heritage Site in 1981.