Mysterious Sinkholes of Mt. Baldy

The Mount Baldy sand dune in Michigan City, Indiana was already pretty interesting, as far as sand dunes go. 

At 123 ft. tall, the "wandering dune" is the tallest dune on the south side of Lake Michigan, moving a little each year as living dunes do. Already tourist attraction, visitors would relax on the beach, take a dip in the lake, and if they were feeling spry, take a sprint up to the top of Mount Baldy to enjoy the Chicago skyline from its delightfully high vantage point. 

Its popularity with view hunters was eventually what made the dune go rogue. The marram grass that stabilized the giant pile of sand was eventually ruined by hundreds of human feet, and the slowly shifting "living dune" became a disturbingly fast moving pile of sand, headed directly for the park's parking lot. 

The park service made the obvious moves—they close the dune off to visitors and replanted the grass. They crossed their fingers and hoped that the vegetation would do its job and slow the moving mountain, but before the marram could root properly, something totally unexpected and still unexplained happened—the dune ate a six-year-old boy. The boy had been walking across a dune when a deep hole appeared out of nowhere beneath his feet, swallowing him quickly and completely. After 3 harrowing hours, the boy was located and rescued, scared and scratched up, but otherwise fine. As the relief from the happy ending to a close call permitted, park rangers and scientists alike turned their attention to the 11 ft. deep hole that seemingly appeared out of nowhere. No one had seen anything like it. With no water beneath it washing away its base, it was particularly unusual. Instead, the sinkhole was created by a tree that had been consumed by the moving dune. After trees are fully submerged by a moving dune they eventually die and decompose thus leaving a hole in the dune that can become a sinkhole in the right conditions. 

After the incident, the beach reopened to the public in 2017 but the dune remains closed to help protect it from further erosion. There are trails at the top of the dune but there is restricted access to these. You can still go on a free, ranger-led hike on this dune to access the incredible views.