An incident 10 years ago this month prompted geological researchers to expose a hazard that nearly claimed the life of a 6-year-old boy.

On July 12, 2013, the boy, from Illinois, was racing his father and two other people up Mount Baldy at Indiana Dunes National Lakeshore (now a national park). The boy disappeared into the sand, and his friend yelled to their dads for help. They could hear him but couldn’t reach him through the 1-ft-diameter hole, and the sand they dug kept rushing back into the void.

Rescuers work to free a 6-year-old boy from Mount Baldy in July 2013.

Three-and-a-half hours later, the rescuers—dozens of people with probes, shovels, and excavators—pulled the boy out. He was still alive: a “miracle on Mount Baldy” which his parents attributed to a higher power.

Geologist Erin Argyilan, who’d been on that dune that day measuring wind speeds, wanted to learn why it happened. Two years later, she and several other researchers published a study in Aeolian Research about dune decomposition chimneys, which form when “migrating dunes encroach on a forest and buried trees subsequently decay, leaving a temporary stable open hole,” the abstract explains. When the stoss, or windward, side of the dune intersects the void, a hazard develops.

Mount Baldy was known to be a migrating and complex parabolic dune; it moves inland about 4 m per year, reforming as the shoreline lakeward of the dune erodes and sediment is redeposited westward. It’s expected to migrate so much that it will overtake a bathroom building and parking area at its base in the next five to 10 years.

What wasn’t apparent until the accident and subsequent dune closure was that Mount Baldy had overtaken and buried an oak forest—and that more holes existed. (“Four holes were discovered when people stepped into them during surveys,” Argyilan reported in the paper.)

Researchers from the IGWS—including then-Assistant Director for Research Dr. Todd Thompson—the National Park Service, and others mapped the locations of 10 holes on the dune, then compared them with a 1939 aerial photo of the same dune. The hole locations were in the former forest and many matched the locations of mapped trees. Additional evidence came from excavating in and around the holes; wood and other organic clues were collected.

Schematic diagram suggesting a possible model for the formation of a deep, vertical dune decomposition chimney. (A) Changes in the position of the buried trees relative to the surface of the stoss slope and exposure of the paleosol. (B) The effects of ongoing vertical erosion of the stoss slope and upward progressing internal decay through time. Areas buried within the dune are represented by the stippled texture. | Argylian et al., Aeolian Research 19 (2015)

As they decomposed with the aid of fungi, the trees’ bark and wood crystallized and cemented with the sand and soil around it. This, along with weathering of the sand, and calcium and magnesium in the water, created the “decomposition chimneys” of calcium carbonate-rich cement, Argyilan et al. wrote.

Similar phenomena had been observed at parks in Oregon and Michigan but had yet to be documented in Indiana. After 175-plus years of geologic research across the state, this was a new discovery, and it led to increased safety precautions at the dunes.

Ten years later, the Summit Trail on Mount Baldy remains closed to visitors except when accompanied by authorized staff. In August 2021, Thompson, Argyilan, and IGWS staff members Matthew Johnson and G. William Monaghan (posthumously) published a paper in the Journal of Great Lakes Research focusing on the internal structure and behavior of the dune through time. They showed that mapping the surface under the dune using ground-penetrating radar and repeated mapping of the dune’s topographic surface using LiDAR can define hazard areas where visitors should not tread.

Argyilan and Thompson recently conducted, with park service consent, a fieldtrip to Mount Baldy for eolian researchers attending the North-Central Section of the Geological Society of America. Such fieldtrips are important in widely disseminating knowledge learned from an unfortunate accident a decade ago.