Crews have quietly launched a $9 million exploratory drilling project at the Millennium Tower to evaluate a planned fix for the sinking and tilting structure, NBC Bay Area has learned.

The project started earlier this month on Beale Street and involves drilling holes between 200 and 300 feet down to bedrock. The goal is to see whether the method will stabilize the troubled foundation.

The building has sunk some 17 inches and is leaning to the north and west. The project to shore up that foundation could cost $150 million.

“There’s a lot of unanswered questions,” Supervisor Aaron Peskin said about the proposed fix for the 58-story concrete building.

NBC Bay Area located holes on Beale Street as the first noticeable sign of the work that began on Jan. 2 and is expected to take 10 weeks when it is officially launched next month.

If test results are promising and the plan passes muster, engineers would bore some 100 to 150 holes through the tower’s 10-foot-thick, steel-reinforced concrete foundation.

Who will pay for that fix is still unclear, as there is a massive legal battle over the building’s troubles.

The so-called micropile strategy is not new; it was used to shore up the Mandalay Bay Resort in Las Vegas, which sank some 18 inches during construction before being stabilized by more than 500 micropiles.

The crews commissioned by the Millennium Tower Homeowners Association will be drilling weekdays from 7 a.m. until 7 p.m. They will be drilling both along Beale Street and in parts of the underground garage.

Peskin says while the testing is a sign of progress, he wants the city to thoroughly evaluate the proposal.

“This kind seems to be dropping out of the heavens,” he says, “and I’m not confident that the city has run (this) through a vigorous peer review process – precisely what was missing when we approved the tower to begin with.”