President Obama, who has been traveling the country urging Congress to pass his jobs bill so Americans can be put to work on infrastructure projects, need only look out a window in the West Wing to see one such project. Under way on his front lawn for the last 17 months, it is highly visible and yet mysterious.

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© Doug Mills/The New York TimesThe project outside the West Wing, said to be renovation of the building’s air-conditioning and electrical systems.
For the first time in recent memory, the ground and pipes beneath the Oval Office are exposed, and each day, workers lower massive concrete blocks several stories underground. Cranes, trucks and construction workers file in and out of the hole, usually through a passageway between the White House and the Old Executive Office Building.

The cavernous hole has inspired much speculation. Is it an Olympic-size swimming pool for the fitness-conscious first couple? A more spacious bunker? A place, perhaps, to hide the deficit?

The General Services Administration says it is an elaborate renovation of the building's aging air-conditioning and electrical systems. These upgrades take place periodically, a G.S.A. spokeswoman, Sara Merriam, said, adding that Washington's Big Dig would soon continue on the other side of the White House grounds.

"As part of this project," Ms. Merriam said in an e-mail, "G.S.A. has been excavating and installing replacement utilities in front of the West Wing and once this phase is complete will sequentially upgrade utilities moving to the North Lawn over to the East Wing."

The final phase of construction will take place near the Presidential Emergency Operations Center, made famous as the spot where Vice President Dick Cheney took shelter in the hours after the 9/11 attacks while his boss remained on Air Force One, awaiting the all-clear to return to Washington.

Although White House officials say they don't know too much about the construction, some have said they think it is, as one put it, "security related."

Another White House official said that the construction would eventually link whatever is being built underground with the operations center, which was originally built during the administration of Franklin D. Roosevelt - the idea being a general expansion of an emergency base of operations.

"It is security-related construction," said the official, who was not authorized to speak publicly about the project. "Even we don't know exactly what."

A larger operations center would reduce the need for emergency evacuations to shadow government locations like Mount Weather in Bluemont, Va., and would help ensure "continuity of government," as Mr. Cheney has put it.

Despite the size of the hole, the controlled silence of the construction workers and the fact that funds were allocated after Sept. 11, 2001, Ms. Merriam maintains it is all business as usual. "The type, size and complexities of work associated with the replacement of the utility infrastructure systems necessitates that extensive excavation and support structures are erected to effectively and safely conduct this work," she said.

Pebble Beach, the unofficial name for the raised platform where television news correspondents do stand-up shots from the White House lawn, offers a clear view of the hole. Camera crews have watched the construction process and regard it with a wink and a nod.

"It's all about air-conditioning, right?" said one when asked what he knew about the work being done.

"Yeah, that's right," replied another, with a smirk. "Air-conditioning."