Everyone knows the Big Dig brought us the Ted Williams and Tip O’Neill tunnels.

But there was one more tunnel, just for pedestrians, built under Summer Street to connect the Boston Convention & Exhibition Center to land owned by the Massachusetts Port Authority.

A tunnel to practically nowhere, it is not much to look at — imagine a 100-foot-long by 22-foot-wide bomb shelter — and is accessible from the basement level of the convention hall. Massport commissioned Big Dig contractors to build the concrete structure for what now seems like a bargain basement price of $1.3 million.

That was around 2001, and today that same tunnel would easily cost more than 10 times that amount.

The passage — finished even before the convention center opened in 2004 — has been waiting for the day a hotel would be built across Summer Street to serve convention goers. That day is finally approaching: Massport this spring picked a joint venture led by Boston developer Davis Cos. and Omni Hotels & Resorts to build a 1,000-room facility on the site. It should be completed in 2021.

“It is a hidden gem,” said Davis executive Brian Fallon after a recent tour of the tunnel.

Massport’s tunnel vision came nearly two decades ago, back when the South Boston Waterfront was a motley collection of parking lots and warehouses. The agency, which owns key parcels between the harbor and the convention center, worried about how friendly the area would be for pedestrians. Rightly, as it turned out.

Massport began a master plan process that led to the decision to build a tunnel so people could exit the convention hall protected from snow, rain, and just plain old frigid New England weather.

At the time, the $15 billion Big Dig was winding down. With Summer Street already torn up and drilling equipment on hand, Massport thought it was an opportune time to ask contractors to squeeze in a mini tunnel.

While the Big Dig spanned three decades, the pedestrian passage took less than six months to construct. Cost overruns plagued the other tunnels, but not Massport’s. Possibly a Big Dig change order that was delivered on time and on budget?

“Absolutely,” said Andrew Hargens, Massport’s deputy director of real estate development, who had just joined the agency when the pedestrian tunnel was being conceived.

Massport and the convention center authority have maintained the no-frills tunnel, which amounts to security officers checking in on the space from time to time. A few years ago, lights were added, and some pieces of plywood were laid down for my recent tour.

The tunnel may look like a Cold War-era bunker today, but by the time the first hotel guests check in and conventioneers stream through, the space will be designed and done up.

By then, other elements of Massport’s plan to make that section of the Seaport more pedestrian friendly will be in place. The agency plans to add a covered walkway along World Trade Center Avenue, which will mean a person stepping out of the convention center using the Summer Street tunnel can walk to the World Trade Center Silver Line stop without getting wet.

What’s left to plan? Of course, a good name for the tunnel.