One of the most dramatic features of the downtown Rose Fitzgerald Kennedy Greenway, the large circular fountain that shoots columns of water skyward, has been dry this spring, even on sweltering days, because of a mysterious mishap.

When the Greenway Conservancy was preparing to inherit the grassy corridor from the Massachusetts Turnpike Authority this February, officials noticed that the fountain's wide stones were wobbling. Their best guess? A truck - possibly a snowplow - mistakenly drove over the fountain last winter, damaging critical underground supports.

"Nobody knows," said Nancy Brennan, executive director for the Rose Fitzgerald Kennedy Greenway Conservancy, the private nonprofit that oversees the park. "I don't know if we'll ever know."

It's another lingering example of the frustrations that accompany almost anything that touches the Big Dig, even the welcome green space that replaced the old elevated highway.

The damage to Rings Fountain occurred sometime between October, when it was turned off for the winter, and February, Brennan said. She does not believe the fountain's lights were damaged, but the park's operations staff will not know for sure until a test drive next week, she said.

Asked why the problem was not addressed well before the onset of the warm weather, Brennan said that it took time to diagnose the problem.

The conservancy, she said, then had to negotiate with the suppliers of the damaged jacks - the 8-inch-tall devices that support the fountain's granite pavers.

The Greenway Conservancy's staff is also working to restore its basin with stronger supports, which will require fewer repairs in the future, and Rings should be fully working by June 19, the nonprofit announced on its website yesterday.

Mario Passi can't wait until the water begins flowing again. The 55-year-old vendor on Boston's harbor walkway likens the fountain to the breathtaking water displays of Las Vegas, especially at night when its waters spray upward in multicolored rays.

"It's free entertainment for zilch, especially during these hard economic times," said Passi, who sells T-shirts and sweat shirts out of his mobile booth, Boston T's Party Waterfront. "It's a shame it's not working."

Tens of thousand of people gathered at the Greenway's official opening last fall, and children reveled in the water that sprays out of the large stones. The 13.2-acre park, which runs from Chinatown to the North End, was a reward to Bostonians after years of construction headaches caused by the massively mismanaged $15 billion Big Dig, which chronically ran over budget and behind schedule.

Once completed, the Big Dig continued to be plagued by problems. It remained without cellphone service long after most major tunnels were wired; and even after it got cell service, the Turnpike Authority has had a difficult time maintaining it. The Greenway has also encountered fund-raising difficulties, and officials have had to repeatedly scale back their vision for the space.

Since the Greenway opened, the park's Rings fountain has been a favorite, serving as a playground for children excited to trigger the water and as a hang out spot for older children.

These days, many people walking along the Greenway ask Charles Gibbs, who volunteers as a gardener, when the fountain will come back on, he said yesterday. Gibbs, a 74-year-old physician originally from New Mexico, moved to the city three months ago and walks through the public space everyday. But he has seen the fountain squirting water only in advertisements, he said.

"I can't wait until the water comes on," Gibbs said. "I want to see this park in full action."

Noah Bierman of the Globe staff contributed to this report.