Kaumana Caves State Park is a small park west of Hilo. Besides the usual park facilities, the main attraction is the caves. The caves are actually a lava tube, created by a flow from Mauna Loa in 1881. They’re accessed through the large opening where a section of the tube collapsed, so the two caves are at opposite ends of this opening.
My understanding is that the caves go on for quite a way, but one is not supposed to go much beyond the entrances because it is, officially, private property after that. It’s also very dark and claustrophobic, so that was enough for me.
The photos show – Above: A view from mouth of the southern cave; Middle: The staircase down to the caves; Below: Foliage that’s grown in the open portion of the tube. The cave entrances are the dark areas at the edge of these photos.