Alien Stele*
Reportedly, this was left behind long ago. No one is exactly sure when it first appeared. Over the years the authorities made various attempts to dig it up and remove it. Regardless of what they tried, they failed.
Rather admit the evidence of it's origins and alarm the residents, they resorted to hiding it in plain sight. It was near the water so they built a large decorative plaza and a raised platform to isolate it. There is no plaque or explanation in place. People occasionally walk by, keeping their distance, and wonder.
* This is intended to be a larger project revealing "the aliens among us". They are usually disguised so identification and "capture" of both the aliens, and the artifacts of their existence is no easy task! Many scoff at this effort, but still pull the blanket over their head at 3am and wonder what that strange noise was….