(CNN) Millions of people could be in the dark for days after Hurricane Irma toppled overhead power lines . The delay raises the question of whether an underground electrical grid would have better weathered the storm -- and helped get life back to normal quicker.

Burying power cables, or "undergrounding," makes lines impervious to damage from wind and ice, and harder for would-be attackers to target. But it also can be expensive, complicate repairs and subject infrastructure to flood damage, experts say.

"You can't say undergrounding is all good or undergrounding is all bad," said Theodore Kury , director of energy studies at the University of Florida's Public Utility Research Center . "It's something that needs to be looked at on a case-by-case basis."

Undergrounding can prevent wind damage ...

Undergrounding entails digging trenches, laying power lines in insulated conduits, then burying them. It's the norm in many downtown areas, and other communities in Europe and the United States have done it; many more have studied it.

In some places, residents and public officials think it's the wise thing to do.

The effort, now enshrined in law, targets the 30 worst-performing electrical distribution lines, he said.

"There is going to be an immediate and significant improvement in the resiliency of the system of those particular lines," Smith said. "They will be able to better withstand calamitous weather in the future, so you won't have to worry about wind or rain or ice or snow."

Kury took a similar view: "If you're more concerned with wind events and flying debris, it may make more sense to bury lines."

.. but it can be pricey.

"It's very handy to say, 'Let the utility pay for it.' But what people forget is that utilities don't pay for anything," Kury said. "It's the customers that pay for the expenses."

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It's incumbent upon utilities and regulators "to make sure when we are asking customers to shoulder an expense, it is providing commensurate benefits," he said.

Washington's plan, pared back from its initial goal and now estimated at $500 million, would cost the average residential customer an additional $1.17 per month, Smith said.

After a 2002 snow storm left about 2 million customers in North Carolina without electricity, a disaster preparedness task force studied the option of undergrounding the state's entire power distribution system.

Replacing overhead lines, the task force found , would take a quarter century and $41 billion, increasing average residential customer bills by 125%. Though less likely to fail in a snowstorm, routine repairs to buried lines would take as much as 60% longer , since problem areas are harder to find and the work requires digging equipment.

Ultimately, the task force opposed converting the state's entire system but urged utilities to research isolated pockets where undergrounding might be cost effective.

Buried lines can pose a flood risk ...

Besides their price tag, buried lines aren't always the best fix in places prone to flooding, including coastal zones threatened by storm surge.

Subsurface flooding, particularly by saltwater, can damage underground lines, according to Entergy Corporation , which provides power service in Arkansas, Louisiana, Mississippi and Texas.

In Gainesville, Florida -- which is inland and not prone to flooding -- power lines largely are buried, said Kury, who calls the college town home.

"If you're primarily concerned with storm surge and flooding, then it may very well make sense to keep the wires above ground," he said.

Indeed, about 60% of the electrical system for Florida Power & Light , the nation's third largest utility, is located above ground. The provider serves more than a dozen coastal counties, wrapping the state from the Georgia line to Tampa.

... but they also can impede attacks.

Extreme weather may not be the only impetus for burying power lines, said Roger N. Anderson, a senior research scientist at Columbia University's Center for Computational Learning Systems

"The country is at high risk, not just from hurricanes but also from terrorism," said Anderson, who has worked with Con Edison to create smart grid systems meant to detect electrical problems before they balloon.

Much of the nation's critical infrastructure, including water, sewage and natural gas lines, already are underground, and the location serves as a key protection against attack, he said. Keeping life as stable as possible is the goal, he said, even when disaster strikes.

"We worry in the homeland security world about anything that lasts more than 72 hours," Anderson said. "That's when the food spoils in the refrigerator and freezer and people start getting desperate."