The explorers who push into uncharted passages regularly say there is no end in sight — that the system has no obvious geological reason to stop, and that the total, when it is finally known, may be far greater.

Four hundred and twenty-six miles.

The coastline of Washington state is roughly 400 miles.

Mammoth Cave is longer.

The system has five distinct levels of passages, formed as the Green River — which flows through the park above — cut progressively deeper into the limestone bedrock over millions of years.

Each time the river deepened its valley, it left behind a level of cave passages that the water had previously occupied.

The upper levels, where the ancient Indigenous miners worked, are the oldest and the driest.