LOUISIANA: Two 20-foot-tall earthen mounds located on the campus of Louisiana State University in Baton Rouge are among more than 800 such mounds built by Indigenous peoples in the state. Radiocarbon dating indicates that they are also the oldest built structures still standing in North America. The older mound, which was likely used for ceremonies, was constructed over millennia, beginning around 11,000 years ago. Both features seem to be aligned with Arcturus, one of the brightest stars visible from Earth.