Emesine lava tube
One of the things to look out for on the Powerline Trail, off of Saddle Road, is Emesine lava tube. There’s a sign right next to the trail, though it could be missed if you happened to be looking the other way while walking that stretch. Bear in mind that if you want to explore the lave tube, the sign indicates that you should first get a permit from the State Department of Land and Natural Resources (Division of Forestry and Wildlife).
These photos are from the entrance of the eastern part of the tube, which is the most accessible. The ceiling here is about 4 feet so it’s not for the claustrophobic. Not sure whether it opens up further along, but be sure to take plenty of good lights.