The beginning is a rocky and uneven with lots of stairs. My kids had headlamps on, but they rent lights that are much better. You don't have to go all the way to the end, where the ceiling meets the ground. You can turn around whenever you want.
On the day we went there were young children who made it with no problem. Its un-guided and you can turn back whenever you want.
The hike is about a mile in and a mile out, totalling just under two miles. The ground isn't always even, but it it's fairly flat. Keep in mind that it's cold in the there and you will need long clothing.
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