In Portal lore, Cave Johnson acquired a disused salt mine in Michigan.
More specifically, the mine was in Michigan’s Upper Peninsula. Cave Johnson used the property as the foundation for Aperture Science’s expanding research complex, building a sprawling underground network of offices, laboratories, elevators, test chambers, and maintenance areas. This location explains why so much of Aperture’s history is buried beneath layers of abandoned infrastructure.
Portal 2 reveals the older facility while Chell travels through long-sealed sections of the complex. The mine’s immense underground spaces also provide the setting for Cave Johnson’s recorded messages, which recount Aperture’s changing test subjects, commercial failures, and increasingly reckless research program.
The state is sometimes confused with Ohio because the lore mentions another Aperture location associated with Cleveland. That does not change the answer to this question: the disused salt mine purchased by Johnson was in Michigan, specifically the Upper Peninsula. The mine is a fictional part of Aperture’s backstory, although Michigan’s real salt-mining history helps the setting feel plausible.