SAN FRANCISCO (CBS SF) — For a look at how San Francisco’s biggest dig since BART stands, you have to ride in a crane-carried bucket about 100 feet beneath the streets of Chinatown.

“Look, this is awe inspiring to be down here,” said San Francisco Mayor Mark Farrell, standing in the giant tunnel. “You know, as mayor I have not had a chance to come down here, to tour the Central Subway before, but to get a tour of what is going on, to see kind of where we are in terms of the construction process, it’s incredible what’s in front of us here.”

Farrell and transportation leaders were doing some Central Subway cheerleading this week at the project’s Chinatown station, now the deepest building excavation in city history.

“We’ve hit a pretty big milestone by completing the excavation, which in some ways is the most challenging part cause you don’t know what you’re going to find,” said Ed Reiskin, Director of Transportation at the San Francisco Municipal Transportation Authority.

One thing construction crews found here was a delay. While the Yerba Buena and Union Square stations were basically giant trenches that were built up to street level, the Chinatown station was burrowed out through a relatively narrow incision on Stockton Street.

It was a trickier dig that took longer than expected, setting the project back by about one year.

“Look, it is what it is right now,” said Farrell. “Obviously our crews are working a significant amount of hours to make sure that we get it done as quickly as possible.”

Despite the delay the city says the project is still on budget, and the work left to be done should be a lot easier than the digging that got them here.

“The next thing we do is the waterproofing,” said Alber Hoe, the Acting Program Director. “Then we start to pour the concrete for the platform. So in essence you could say we’re probably halfway done on this station.”

So the station is halfway finished. SFMTA says completion and fare service is expected in December 2019, but there is already talk of taking this project even fartherm, North Beach, or beyond.

“Very thankful that they had the foresight to already to dig the tunnels to North Beach,” added Farrell, speaking in support of a 3rd phase of the project. “So if and when we can do that we want to do that as a city. And I really do think as a city.”

“We’d would need to find a place for a station,” said Reiskin.

Then we would need to find what the path would be if we were going to continue to Fisherman’s Wharf. We did a feasibility study on that a few years ago, there’s a lot of different options for getting there.”

There is currently no money for that extension, and the existing 1.7 mile project, from South of Market to Chinatown, comes with a price tag of $1.6 billion.

Critics have long said that money could have carried more people to more places via cheaper alternatives like bus rapid transit, now coming to streets like Van Ness and Geary, but there is a growing consensus among city planners that the gridlock on city streets will eventually require solutions below them, no matter what the cost.

“We need to think generations ahead,” said Mayor Farrell. “As we have more and more people living in San Francisco, working in San Francisco, on cars, on bikes, on foot, the more that we can put traffic we underground the better we’re going to be.”