### Mima Mounds
USA — Thousands of grass-covered domes make this prairie look like giant bubble wrap, built by earthquakes, glaciers, or gophers.
When explorer Charles Wilkes stumbled onto this Pacific Northwest prairie in 1841, he assumed the thousands of perfectly spaced earthen domes were Native American burial mounds. He excavated several, but found absolutely no human remains or artifacts. The landscape wasn't an ancient graveyard; it was an entirely natural geological puzzle sitting on an outwash plain carved by the ancient Vashon Glacier roughly 15,000 years ago.
Each mound is a massive pile of dark, gravel-free topsoil resting on a bed of heavy glacial cobble. Ranging from one to seven feet tall and up to 40 feet across, they are so closely packed that the landscape looks like a sea of giant, grassy bubble wrap. Because they are strictly natural, geologists have spent over a century trying to figure out what built them.
Today, scientists debate three main hypotheses. The Seismic Theory suggests that massive earthquakes shook the unstable, waterlogged glacial soil, vibrating the dirt into neat clumps. The Glacial Melt Theory argues that retreating ice or wind deposited the sediment into organized piles. Finally, there is the Pocket Gopher Theory: the idea that thousands of years of tiny rodents constantly pushed dirt upward to build dry homes above the flooded rocks, eventually engineering a sprawling labyrinth of domes.