Deep underneath the Fraser River hides an engineering marvel — the biggest water infrastructure project ever attempted in Metro Vancouver — and it's almost ready.

The tunnel, 3.5 metres across and one kilometre long, runs under the river between Surrey and Coquitlam, and will soon transport drinking water to Surrey and other communities south of the Fraser.

"This tunnel is recognized throughout the world in the tunnelling community as being significant and having challenges that haven't necessarily been faced by other projects," said Project Manager Tim Langmaid.

The Fraser River drinking water tunnel

A tunnel engineer leads the way, deep underneath the Fraser River. (CBC)

The tunnel, which sits 30 metres below the river, cost $240 million and took four years to build. The biggest challenge came when the massive tunnel boring machine nicknamed "The Squirrel" broke down from wear and tear.

After breaking through boulders the size of cars, it gave out 200 metres shy of the finish line. Crews had to repair it on the fly.

"The best case scenario was that the tunnelling would take three months and what actually happened was it took 16 months," said Langmaid.

In July, the project hit a major milestone — The Squirrel broke through on the Coquitlam side of the river.

Major earthquakes, rough water

Murray Gant, Senior Engineer with Metro Vancouver, say the new tunnel will have double the capacity and be much more durable than the existing pipe, which was damaged by unusually rough water nearly 20 years ago. 

"The nice advantage about this project is we're able to put the tunnel deep so it will withstand river scour, a major earthquake and it will provide for major population growth."

The Fraser River drinking water tunnel

The end of the line - tunnel engineers continue working to install the welded steel main. (CBC)

The toughest part of the job is over, but there is still work to do — crews need to install the welded steel water main. 

If all goes as planned, drinking water will be flowing through the tunnel late next year.