ROCKLAND, Maine — The state Department of Transportation said Wednesday that it will pay the entire cost of repairs associated with a large hole that opened on Old County Road this week.

“We’re considering this capital work because of the immensity of the situation,” said David Bernhardt, the department’s director of engineering and operations.

Bernhardt said he did not know what caused the 25-foot-wide hole, which grew from a pothole Monday night and was about 50 feet deep by Wednesday.

A member of the city’s Department of Public Works said Tuesday the road was built over a tunnel connecting two old limestone quarries, and the supporting structure over the tunnel let go.

No one was injured and traffic has been detoured to Route 1 until the road is repaired.

Bernhardt would not speculate on what sort of work needs to be done, how much it will cost or when it might be done. He said hydraulic, environmental and other problems complicate the matter and he should have an estimate in the next couple of days.

“It [the work] is basically filling it in, but you have water going through there and you can’t dam it up. The water has to go somewhere,” he said.

“We’re not sure what caused it. There are a lot of heavy trucks on the road because of the quarries,” he said.

Old County Road is a state aid road in an urban compact, which means the city is responsible for routine maintenance. Because the work that will need to be done is so extensive, the town will not have to pay for it, officials said.

“It is a fabulous response to this very dangerous situation that is definitely not a regular maintenance issue,” said Rosemary Kulow, Rockland’s city manager. “We’re extremely happy that MDOT stepped up to the plate and accepted responsibility to help resolve the issue.”

Kulow said the city will be responsible for signs and barriers to keep people away from the hole, which she said has expanded since Tuesday, but was not sure by how much.

State Transportation Commissioner David Cole said: “This is a fix we’re going to have to live with for a long time. Hopefully it is nothing we’ll have to revisit in the next 50 years.”