(CBS) — The Indiana dune where a 6-year-old boy fell and became trapped for hours in crushing sand has been closed indefinitely, the National Park Service announced.

The closure of Indiana Dunes National Lakeshore’s Mount Baldy was extended to “bring in additional resources for the investigation of the hole that opened up on July 12,” park officials said.

Authorities at the Indiana Dunes National Lakeshore meet tomorrow to determine the next steps as they investigate last Friday’s incident in which a six-year-old boy was critically injured after sinking under the sand and being trapped for more than three hours on Mount Baldy.

The cause of the incident has baffled people at the National Lakeshore. Bruce Rowe is a park ranger who has studied dunes.

“It truly is a mystery to us at this point. We’ve never seen a hole open up on a dune , but we are searching the scientific literature right now to see if such a thing has happened anywhere in the world,” said Rowe.

A working theory is that an old tree under the sand might have rotted; leaving a void that may have collapsed when the boy was swallowed up.

“Where they found the boy, there was some rotten, what appeared to be tree material. Whether it was from the trunk or the bark or what, we are not sure,” said Rowe.

It was not a sinkhole he says, not by the technical definition.

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