We pulled over as well and discovered that they were admiring a large cave that had formed when a lava tube had collapsed. (It must have collapsed years if not decades ago.)

The area around the cave and the hole in the roof of the cave located further inland were marked with white spray paint to indicate where visitors could safely walk.

I wish I could tell you exactly where this quick and free attraction is located, but I only know that it is in a lava field just north of Kailua-Kona on the right (east) side of Hwy 19.