Normally, standing in this room would be like looking up from the bottom of a deep, deep swimming pool.

Today, it's bone dry.

This is one of six settling basins, a key part of how the historic R.C. Harris Water Treatment Plant turns Lake Ontario water into the drinking water that comes from your tap. The basins are currently undergoing a multi-year, $22 million rehabilitation project — the first upgrade some have seen in over fifty years.

Toronto Water's Lou Di Gironimo, right, gives Mayor John Tory a tour of the facility, which provides a quarter of the city's drinking water. (John Rieti/CBC)

"For this plant, it's one of the larger projects you would see," said Lou Di Gironimo, General Manager of Toronto Water.

R.C. Harris has been in operation since 1941 and provides around one-quarter of Toronto's drinking supply.

Opened in 1941, the design of R.C. Harris makes it feel like a temple of water treatment. In this part of the facility, the water is being filtered. (John Rieti/CBC)

Giving Mayor John Tory a tour, Di Gironimo proudly explained how water can be pumped from the east-end facility all the way into York Region.

The basins, he said, play a key role. Here's how they work:

Lake Ontario water is pumped into the facility.

Screens remove large debris, and chemicals like chlorine and alum are added.

The treated water then sits in the basins for several hours, allowing sediment to sink to the bottom.

Water is then filtered, treated with more chemicals and then stored.

The water gets pumped out across the city.

Tory told reporters he considers the process a "miracle" of city living.

"We're lucky to have clean running water, compared to many other places in the world, that we can drink right out of the tap — we don't even have to think about it," he said.

R.C. Harris pumps water across the downtown core, and as far north as York Region. (John Rieti/CBC)

Tory said R.C. Harris, which is also renowned for its architecture, plays a crucial role in the city. While the plant can close for maintenance, it has been running 24/7 throughout the rehabilitation work.

That work includes adding a new waterproof membrane to the basins to stop groundwater from seeping in, as well as maintaining the structure of the giant tanks. There are also smaller fixes, including access hatches and ladders, while the area around the plant also needs some work.

Part of the R.C. Harris grounds will also be upgraded as part of the $22 million project. (John Rieti/CBC)

Di Gironimo said the plant's design is so good, that it doesn't need a major overhaul.

"We're just actually proud of restoring it back to its original character and making it run as effectively as it was the first day it was put into service," he said.