Provide a detailed summary of the following web content, including what type of content it is (e.g. news article, essay, technical report, blog post, product documentation, content marketing, etc). If the content looks like an error message, respond 'content unavailable'. If there is anything controversial please highlight the controversy. If there is something surprising, unique, or clever, please highlight that as well: Title: 9k-Year-Old Stonehenge-Like Structure Found Under Lake Michigan Site: www.thearchaeologist.org Some may be natural while others appear of human origin, but those forming what could be the petroglyph stood out, Holley said. Viewed together, they suggest the outlines of a mastodon-like back, hump, head, trunk, tusk, triangular-shaped ear and parts of legs, he said. “We couldn’t believe what we were looking at,” said Greg MacMaster, president of the underwater preserve council. Specialists shown pictures of the boulder holding the mastodon markings have asked for more evidence before confirming the markings are an ancient petroglyph, said Holley. “They want to actually see it,” he said. Unfortunately, he added, “Experts in petroglyphs generally don’t dive, so we’re running into a little bit of a stumbling block there.”