SEPTA introduced Tuesday the first stage of its multiyear, multimillion-dollar project to remake the tunnels beneath Center City into a more attractive pedestrian concourse through the heart of Philadelphia.

"SEPTA long saw the potential to transform these corridors into a place with a clear and welcoming identity," said Jeff Knueppel, the transit agency's general manager.

The white-tiled, concrete-paved corridors radiate out from a redesigned "oculus," a circular space that once had been open to the southwest corner of then-Dilworth Plaza about 10 feet above. The new oculus is lit with orange light, and the corridors are lined with digital screens. In total, 22,000 square feet of space has received a facelift.

The $10 million work on the oculus and corridors began in earnest about two years ago, said Bob Lund, SEPTA's assistant general manager of engineering, though before that structural repairs had already been done to repair leaks. The space was open to transit riders within the past month, SEPTA officials said.

In the years to come, SEPTA anticipates spending $59 million renovating the platforms, stations, and miles of corridors that run beneath the city, from Eighth Street to 18th Street along Market Street, and from JFK Boulevard to Spruce Street on Broad Street. It includes 123 head houses, stairways, and entrances, Knueppel said, covering about 320,000 square feet total and connecting both subway lines, Regional Rail, PATCO, and the trolley system. SEPTA took ownership of the underground space from the city in 2014.

When the Gallery on Market Street reopens as Fashion District Philadelphia, it will be possible to cover all that ground on foot without going above ground, something officials said is a safer alternative for pedestrians than walking along the street.

The next space to be renovated will be the corridors from Juniper Street to 11th Street, which will be put out to bid in the summer, and then the corridors beneath the Municipal Services Building and LOVE Park, which will be bid in the fall, Lund said. The work is expected to take two years.

More difficult is the plan to renovate the walkways beneath Broad Street. SEPTA is negotiating with businesses and community groups to determine what access to provide, but Knueppel envisions that space as one with "enormous possibility," including potential for shops, dining, performance space, underground gardens, and bike parking.

It will likely take 12 years before the work of renovating all the subterranean space is finished.

The SEPTA renovations are a complement to the work done at Dilworth Park in 2015 and, more recently, LOVE Park, said Paul Levy, president and CEO of the Center City District.

"The whole idea of these kinds of investments is, you drop the seed in the ground," he said, "and it grows."