Scientists hoping to detect dark matter deep in a former South Dakota gold mine have taken the last major step before flipping the switch on their painstakingly delicate experiment.

Harry Nelson, a University of California, Santa Barbara physics professor and a principal investigator on the Large Underground Xenon experiment, says the team has finished submerging its phone booth-sized detector in a 70,000-gallon vat of purified water. The process took more than two months.

He said Monday that the team could be ready to begin collecting data by February.

Scientists know dark matter exists but haven’t been able to detect it. Regular matter accounts for about 4 percent of the universe’s mass, dark matter accounts for about 25 percent and the rest is mysterious dark energy.