They hide quietly beneath your house, crisscrossing under neighborhoods, roads and buildings, just waiting to burst and give you a headache.
More than 5,500 miles of Washington Suburban Sanitary Commission pipes are buried beneath Montgomery and Prince George's counties, and with winter settling in, officials are gearing up for breaks, leaks and bursts.
Winter, WSSC spokesman Jim Neustadt said, is when underground water and sewer lines break or leak more frequently.
The temperature of the river water varies, causing stress on the pipes, he said.
That's why he was in a Gaithersburg neighborhood Tuesday morning with WSSC General Manager Jerry N. Johnson and other officials as work crews dug up and replaced a problematic water main.
The work is slow. To replace three quarters of a mile, crews have been working on this pipe here on Stonebridge Drive since Oct. 11.
Customers had been complaining about a high number of breaks, so the work is necessary, Johnson said.
Still, WSSC prepares for winter with more crews on standby to respond to emergencies and plenty of money set aside to pay for overtime, he said.
"We prepare to get out and do what have to be done," Johnson said.
Last year there were nearly 2,100 broken and leaking mains, with about 30 percent of those (647) happening in December. The average from 2000-2010 was 1,742 per year, officials said.
This year promises to be no different. Neustadt said there have been more than 150 so far this month alone.
Part of the problem is simply aging infrastructure, Johnson said. A quarter of all the pipes in the ground are more than 50 years old.
Some mains last up to 70 years, while others barely make it 30, he said. They break or leak because of the material used to make the pipe, or nearby construction, or even the kind of soil covering the pipe.
"We're well behind the curve," Johnson said.
The main on Stonebridge Drive in Gaithersburg crews were working to replace today is part of the 41 miles of water main WSSC is scheduled to replace in fiscal 2012.
The pipe here was buried in 1976.
And although WSSC isn't responsible for indoor plumbing, officials are offering tips on how to keep their pipes and water meters from freezing this winter.
They suggest insulating outside walls, insulating pipes and meters, seal all leaks in crawl spaces and basements, turn off water to outside faucets, remove hoses and drain the pipes.
If a pipe freezes, completely open the cold water faucet nearest the frozen pipe. This will relieve the pressure and reduce the chance of breakage. Or you can use a hand-held dryer to thaw the pipe yourself.
WSSC serves 1.8 million people in Montgomery and Prince George's counties and has a $1.192 billion budget in fiscal 2012.