The first of three holes will be bored into the bedrock and equipment will then be lowered into the holes to measure movement.
RELATED: SF skyscrapers to be examined following Millennium Tower tilting
Engineers hope data will then show why the 58-story tower has sunk 16 inches and tilted two inches since it was completed in 2009.
Experts only expected it to settle four to six inches in all.
The data collected by the soil engineers could ultimately determine who's to blame and who could possibly be forced to pay millions to solve the problem.
The testing is expected to take a month and then we may get some conclusions in five to six months.
WATCH VIDEO: Luxury skyscraper Millennium Tower sinking in downtown SF
Click here to read the subpoena formally filed by San Francisco City Attorney Dennis Herrera.
HOA engineer is same one who said earlier Millennium Tower could sink as much as 24 to 30 inches based on test samples collected by others. pic.twitter.com/Bieuknu75Y— Vic Lee (@vicleeabc7) September 26, 2016
HOA of Millennium Towers begins soil and structural testing on tilt and sinking of skyscraper. pic.twitter.com/kG9cdT4G4R— Vic Lee (@vicleeabc7) September 26, 2016