Cook County sheriff's deputy Ed Graney was on routine helicopter patrol when he noticed towering lime green plants amid the dark weeds along the Bishop Ford Expressway.
They were marijuana plants, row after row over an area the size of two football fields near Stoney Island Avenue and 105th Street.
"We had the right altitude and the right angle, the right sunlight and happened to be glancing down," said Graney, a tactical flight officer for the sheriff's department.
He and the helicopter pilot, Chicago police officer Stanley Kuprianczyk, spotted the pot around noon Tuesday and narcotics officers moved in to secure the area.
"The plants are pretty big. They're as big as Christmas trees," Chicago police Lt. Michael Ryle said. "Some of the plants are as tall as 10 feet tall."
There was also a makeshift "watchman" area where the still at-large growers could keep an eye out for trespassers, Ryle said.
The growing area is "very secluded, very difficult to see from anywhere but the air," Ryle said. "You can't smell it from here, but the smell is very intense once you're inside.
"There's some evidence in there of like a camp, where there's been a watchman. . .or a guard sitting on it."
Police Superintendent Garry McCarthy said it's unclear who owns the land.
"The brazenness of it is. . .people will do whatever they can to make their money," McCarthy said. "I think that spending that much time and effort setting up an operation like this in a place like Chicago, it is brazen."