A boulder the size of a small garage has turned up 30 feet beneath O'Hare International Airport's airfield, and Chicago wants the public's help giving it a name. The 50-ton rock, roughly 11.5 feet wide, 8 feet deep and 7.5 feet tall, was uncovered during excavation work tied to a massive stormwater project, and the Chicago Department of Aviation is now inviting residents to submit name ideas before the city puts five finalists to a public vote.
The boulder surfaced during digging on the South Detention Basin stormwater detention project, work that falls under the airport's sprawling, multibillion-dollar modernization effort, according to the Chicago Tribune. A project geologist has identified the rock as dolostone, a sedimentary carbonate rock composed primarily of dolomite, or calcium magnesium carbonate, as reported by CBS News. The Tribune notes dolostone is closely related to limestone, and CBS News adds that it forms when magnesium-rich groundwater alters limestone by replacing calcium ions with magnesium.
Aviation Commissioner Mike McMurray said airport staff had already given the rock an informal nickname before the public contest even launched. “We've been calling it the O'Hare bean,” McMurray said, according to the Tribune's reporting. He also offered a pun-friendly assessment of the discovery, saying “the world already knows that O'Hare rocks,” and argued that “a rock of this magnitude deserves a name of its own.”
How the Naming Contest Will Work
The public can propose names for the boulder through an online form over the next three weeks, per the Tribune. From there, the Chicago Department of Aviation will narrow submissions down to five finalist names, and the public will then vote to pick the winner. The department has used this crowdsourcing format before: the city relied on a similar process to name its snowplows.
That snowplow-naming tradition has become something of a Chicago institution. Once the boulder has its name, CBS News reports it will be featured in a permanent airport exhibit and on O'Hare's official social media channels.
The Stormwater Project That Unearthed It
The boulder's discovery is a byproduct of serious infrastructure work happening far below O'Hare's runways. The South Conveyance Project relies on a 400-foot-long tunnel boring machine operating 50 feet underground to build an 8,000-foot-long, 14.5-foot-diameter stormwater tunnel, according to Construction Equipment Guide. The $137 million tunneling effort is designed to redirect stormwater from the Central Detention Basin to the South Detention Basin.
That construction is part of what the Tribune describes as the city's multibillion-dollar O'Hare modernization effort, which broke ground last August and is expected to be completed in 2034, with a total cost of $8.8 billion. City of Chicago records show the program, known as ORDNext, secured a $1.455 billion guaranteed maximum price cap with lead contractor team AECOM Hunt Clayco Bowa for the upcoming 19-gate Concourse D, slated for completion in late 2028 as O'Hare's first major new terminal build in more than 30 years. Chicago YIMBY additionally reports the department updated its ORDNext timeline to break ground on Phase 1 of Concourse E later this year, adding 14 gates.
A Rock 400 Million Years in the Making
Dolostone isn't just an obscure geological curiosity in Illinois — it's the state's official rock. WTTW News reported in 2022 that dolostone earned that designation after a push by suburban elementary and middle school students, honoring bedrock formed during the Silurian period roughly 400 million years ago, when the Chicago region was covered by a tropical inland sea. That same bedrock forms the foundation beneath glacial soil across most of northern Illinois, per WTTW News.
The region's Silurian bedrock lies beneath glacially deposited soil. All of this activity is unfolding directly beneath one of the country's busiest airports.
Public Whimsy Meets Corporate Dollars
The lighthearted rock-naming contest is a public-facing initiative from the Chicago Department of Aviation.
