A piece of Toronto’s past was unearthed Sunday, courtesy of a summer rain storm.

That’s when construction workers excavating the back lot of a Staples office supply store in Leaside ruptured an underground fuel tank — a remnant of a First World War airfield used to train pilots, mechanics and maintenance crews for the Royal Flying Corps.

The punctured tank didn’t raise any eyebrows at first. The construction company made arrangements to have the contents pumped out yesterday.

But then the rain came, kicking off a chain of events that led to the decades-old chemicals, including aviation fuel and kerosene, seeping into the sewer system and eventually into the waters of the Don River.

By yesterday, the fuel spill had been contained by a series of booms and the tank taken away, but Toronto Fire Capt. Mike Strapko said his hazardous materials team was still a little shocked over what had been uncovered.

“It’s routine for us to control these kind of spills that get into the waterways and sewer system ... but this one has been a little unusual,” he said. “It could have been much worse.”

The airport’s biggest claim to fame came in 1918 when Brian Peck safely landed the country’s first airmail flight there.

What authorities didn’t realize at the time was that Peck had used the delivery as a guise to transport several cases of liquor from “wet” Montreal to “dry” Toronto at the height of prohibition.