Police arrived at 3014 N. 7th Street shortly before 1:00 p.m. on Wednesday.
Inside they found what's being called an "elaborate" hydroponic marijuana farm.
In all, police say, they recovered $5 million worth of the drug.
"It's a place where all they're doing is growing marijuana. There's no furniture, no place to relax, no television, no entertainment. They were growing it in this spot and harvesting it in this spot," said Capt. Debra Frazier of the Philadelphia Police.
To hide the operation, the growers put lumber and black tarps over the windows to hide the high-intensity lights.
Neighbors say they didn't have a clue.
"You never saw anybody go in and out of there," said neighbor Madelyne Nise. "So I don't know how they got that in there."
But, apparently, someone got wind of what was going on, after detecting a strong smell of marijuana, and called police.
No one was inside when police raided, and no one has been arrested. However, police do have a person of interest in mind.