Three stories under Queens Boulevard, with thousands of cars zooming by overhead, a “mole” is boring a new sewer tunnel.

Instead of cutting an open trench across six lanes of traffic, closing the busiest street in Queens, the city instead is using the same engineering technology that built the rail tunnel below the English Channel.

Last Friday, a huge tunnel boring machine — called a TBM for short — was lowered into place down a deep shaft on the corner of 69th Street and Queens Boulevard.

This week, moving at a speed of 30 to 40 feet a day, the computer-controlled TBM is cutting a circular hole, 6 feet wide, directly under the crosswalk from the south side of the boulevard to the north.

The storm sewer is the second part of a troublesome sewer rebuilding project in flood-prone Maspeth that began six years ago.

The open-cut dig along Calamus Avenue dragged on for years, cracked foundations and left residents without water.

A year and a half ago, the city Department of Design and Construction decided to rework the plan that required an open excavation across Queens Boulevard and call for the TBM, said Ali Malick, the assistant DDC commissioner for northern Queens. “This was faster, cheaper and more environmentally sound,” he said.

The unusual $2.5 million operation began full-scale on Monday. A score or so of city officials left their offices to watch as pipe was hydraulically pushed into the tunnel behind the machine to shore up the walls.

If all goes according to plan, the TBM will finish eating through 300 feet of rock, dirt and sand on Saturday, officials said.

But the unexpected has already reared its head.

A boulder the size of a city bus was discovered three weeks ago directly in the path of the machine.

That’s too much for the TBM’s drill bit to handle. So on Monday, while it was boring below the south side of the street, a crew of about a dozen men was furiously digging out the boulder on the north side.

“We’ll get it,” said a worker climbing out of the hole.

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