Which government project does Hopper discover while investigating Hawkins Lab in season 1?

The government project Hopper discovers while investigating Hawkins Lab in season 1 is Project MKUltra.

Hopper’s investigation begins after Will Byers disappears and the police chief notices suspicious evidence connected to Hawkins National Laboratory. As he digs into the laboratory’s history, he finds links to MKUltra, the real CIA program that researched mind control, interrogation, and the effects of psychoactive drugs. The series uses that historical program as the foundation for Dr. Martin Brenner’s experiments.

The storyline connects MKUltra to Terry Ives, Eleven’s biological mother. Terry participated in secret experiments while pregnant, and the show implies that Brenner’s work contributed to Eleven’s unusual psychic abilities and eventual abduction. Project MKUltra is not the same as Project Blue Book, which investigated UFO reports, or Project Manhattan, the World War II atomic-bomb program. In Stranger Things, MKUltra explains the laboratory’s human experimentation and the origins of Eleven’s backstory.