
This is the mesmerizing moment a 21ft-wide, 800,000lb tunneling machine broke through into what will become one of Seattle's newest subway stations when it is completed in 2021.

The machine, named Brenda, has spent the last 16 months excavating 200,000 tons of soil from underneath The Emerald City as it bores a four-mile-long tunnel as part of a $2billion construction project.

Brenda will now spend the next six weeks undergoing maintenance and repairs at the station, where there is access to the surface, before carrying on drilling to the Husky Stadium, another couple of miles to the south.

Brenda is different from, and much smaller than, Bertha - the world's largest boring machine - which is supposed to be excavating a two-story highway along Seattle's waterfront, but has been stuck for two years.

This is the moment a 21ft-wide, 800,000lb drilling machine named Brenda punched through the wall of a new subway station underneath the streets of Seattle

Once it reaches the Husky Stadium, due to take place early next year, its huge drill-face will be removed along with its delicate interior, while the outer shell will be built into the wall of the tunnel.

While Brenda, named after the project manager's wife, is digging the southbound tunnel, her sister machine, Pamela, is drilling the northbound tunnel, having started four months behind.

BRENDA BY NUMBERS Crew: 10 Width: 21ft Length: 300ft Weight: 800,000lbs Top speed: 40ft per day Operating depth: Between 15ft and 130ft Advertisement

Pamela is due to finish cutting the second tunnel later next year, before a extensive series of safety tests need to be carried out and track laid, with the new subway tunnels due to open in 2021.

Brenda and Pamela are both constructed like giant mechanical worms, with a huge flat drill face at the front, and a long cylindrical body trailing out behind them.

The drill-face is covered with cutting teeth which are designed around the type of soil to be cut through, and filled with holes which allows the soil they cut away to fall through and into a chamber behind the main face.

There are also pipes fitted to the main face which spray a foam substance which helps to cool the working parts of the machine, reduces dust, and reduces friction between the drill-face and the surrounding earth.

Once there it is mixed with other substances to turn it into a paste, and a long corkscrew moves it under pressure towards the back of the machine where it is dumped on to a conveyor belt and moved out of the tunnel.

As the machine inches forward, usually by only a few feet per day, a mechanical arm picks up stacks of concrete slabs, and lays them around the sides of the tunnel walls, stopping it from collapsing.

Brenda has spent the last 16 months drilling more than three miles underneath Seattle at a top speed of 40ft per day, shifting 200,000 tons of earth in order to create a new subway tunnel

Brenda operates like a giant mechanical worm, crushing through soil with its drill-face which is filled with holes, allowing the dirt to fall through where it is turned into a paste, then carried away on a conveyor belt

The concrete slabs are brought in on the same conveyor belt used to move the soil away, and contain special seals which also make the tunnel watertight.

While Brenda and Pamela are both impressive pieces of kit, they are nothing compared to Bertha, the world's largest tunneling machine, which is seven times their size overall, and has a drill face almost double the diameter.

Bertha is supposed to be digging a two-story highway tunnel along Seattle's waterfront, but overheated in December 2013 after digging just 1,000ft of the proposed route, and is still stuck there.

The huge machine broke down after overheating when several protective seals failed, allowing grit and rocks to get inside, damaging key components.

Workers removed the drill-face, which is five stories tall and weighs 2,000 tons, earlier this year, and have since been working to test and replaced damaged components inside. Bertha is not expected to start drilling again until December this year.