The first section of the Second Avenue Subway opens this weekend —  and with a cost of four-and-a-half-billion dollars, it hasn't come cheap. In Part 2 of our series, "A Second on First," NY1 Transit Reporter Jose Martinez looks at why the city's newest and very deep subway extension cost so much money — and time — to build.

From the blasting to the burrowing, building the new Second Avenue subway has been anything but easy.

"It was an engineering marvel of constructing this tunnel and station in a dense populated area," said MTA Capital Construction Program Executive Anil Parikh. "It was very, very challenging."

It took four years to build the first nine miles of the subway in Manhattan more than a century ago.

But the MTA spent more than twice that amount of time — nine years — to construct the much smaller one-and-a-half-mile line that's opening under Second Avenue from 63rd to 96th Streets.

One major difference:  A century ago, subway tunnels were built by digging huge trenches through existing streets... a technique called “cut and cover.”

However, that approach would be too disruptive on the packed Upper East Side of today.

Instead, the new line was born by blasting through bedrock, six stories beneath the street.

"We were mining underground and there were times people did not know that we were mining the tunnel underground," Parikh said.

Another complication: underground utility lines, which did not exist a century ago, had to be shifted.

"We had to relocate so we can construct a slurry wall and start the construction and remove the soil so that we can build the structure from the bottom up," Parikh said.

"You have a whole underground network of mechanical services in New York," said former MTA Chairman Dick Ravitch. "I can't imagine what they had to deal with here."

A 485-ton machine sliced through about 50 feet of bedrock a day

"We were able to mine the tunnel much faster without impacting the community and the buildings associated with it," Parikh said.

Still, businesses suffered in the construction zone. And a botched blast in 2012 sent debris flying eight stories in the air.

For years, these unsightly structures dotted Second Avenue, buffers against noise where construction debris was collected.

"They served a purpose big time," Parikh said. "And that is something, big lessons learned if we were to do this as we go up north to the next section."

If it is built, that next phase will extend the new line north from 96th to 125th Streets — after many more years of costly and complicated construction.