This looks like the skull and crossed bones are made of white lava rock, which is a nice contrast against the black lava rock. I think this could definitely be marking some great buried treasure...skull treasure perhaps? Though I don't recall any tales of pirates voyaging to Hawaii, I guess anything is possible. How would one dig through lava rock to find treasure? A jack hammer? I think the only treasure under here might be more lava...which I don't want to go hunting for, so perhaps this is a warning to not dig and to venture on to the beach.