A mysterious creature recently washed up on the shore of Lake Michigan, photos show, triggering speculation, fascination and the question, what the heck is it?

Robert Loerzel, a photographer and freelance journalist, came across the decomposing animal on Montrose Beach, in Illinois, he said in a tweet.

Whatever it was, it appears to have died quite some time ago. But what remained of it, the serpentine skeleton and scaly skin, provided plenty of fuel for people’s imaginations.

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“I KNEW there were sea monsters in Lake Michigan,” a Twitter user commented on Loerzel’s post.

Read more: The Kansas City Star