Mammoth Cave Lava Tubes

From the USFS website: 

"Mammoth Cave, at 8050 feet in elevation opens to one of the largest lava tubes in Utah with over 2200 feet of passages. Formed by cooling lava and flowing water, Mammoth Cave is part of the Markagunt Plateau. Geologists believe that some of this lava is only several thousand years old – relatively young in geologic terms! During summer months, the cave is a popular nightly resting spot for several bat species, and is frequented by other small birds and mammals.

The cave looks like a big hole in the ground at first glance and is fun to explore. Take a good light for every person. The cave is pitch dark after the first turn. The exit is a small tube that allows only enough room to slide through on your belly. If you are large or claustrophobic turn around and go back the way you came. Water seepage at the end of the cave makes the rocks slippery. Wear appropriate hiking shoes into the cave."

Just east of Duck Creek Village on State Road 14, turn left onto Forest Road 067. Go five miles and then turn right onto Forest Road 064. After about one mile turn left and look for the sign." 

The first/main cave entrance is about 40 yards from the parking area where you can scramble down in. There is a gate inside the cave that opens into the main tunnel. From there have fun exploring!

Given its elevation the cave is often not accessible in winter due to snow.