In the aftermath of violent storms that knocked out power to millions from the Midwest to the Mid-Atlantic , sweltering residents and elected officials are demanding to know why it's taking so long to restring power lines and why they're not more resilient in the first place. The answer, it turns out, is complicated: Above-ground lines are vulnerable to lashing winds and falling trees, but relocating them underground involves huge costs - as much as $15 million per mile of buried line - and that gets passed onto consumers. With memories of other extended outages fresh in the minds of many of the 1.07 million customers who still lacked electricity late Tuesday, some question whether the delivery of power is more precarious than it used to be. The storms that began Friday knocked out power to 3 million and have been responsible for the deaths of 24 people in seven states and the District of Columbia, including a utility contractor who fell to his death Monday in Garrett County, Md. "It's a system that from an infrastructure point of view is beginning to age, has been aging," said Gregory Reed, a professor of electric power engineering at the University of Pittsburgh. We haven't expanded and modernized the bulk of the transmission and distribution network. A Gulf Power lineman works to restore a power line in Middleburg, Va., Tuesday, July 3, 2012. Severe storms swept through the area leaving many homes and businesses without electricity. The powerful winds that whipped through several states late Friday, toppling trees onto power lines and knocking out transmission towers and electrical substations, have renewed debate about whether to bury lines. District of Columbia Mayor Vincent Gray was among officials calling for the change this week and was seeking to meet with the chief executive of Pepco, the city's dominant utility, to discuss what he called a slow and frustrating response. "They obviously need to invest more in preparing for getting the power back on," said Maryland state Sen. James Rosapepe, who is among those advocating for moving lines underground. Every time this happens, they say they're shocked - shocked that it rained or snowed or it was hot - which isn't an acceptable excuse given that we all know about climate change. Though the newest communities do bury their power lines, many older ones have found that it's too expensive to replace existing networks. To bury power lines, utilities need to take over city streets so they can cut trenches into the asphalt, lay down plastic conduits and then the power lines. Manholes must be created to connect the lines together. The overall cost is between $5 million and $15 million per mile, according to the Electric Power Research Institute, Inc., a nonprofit research and development group funded by electric utilities. Those costs get passed on to residents in the form of higher electric bills, making the idea unpalatable for many communities. Power lines are already underground in parts of Washington, but initial estimates are that it would cost as much as $5.8 billion to bury them throughout the entire city and would cost customers an additional $107 per month, said Michael Maxwell, Pepco's vice president of asset management. North Carolina considered burying its lines in 2003, after a winter storm knocked out power to 2 million utility customers. The North Carolina Public Staff Utilities Commission eventually concluded it was "prohibitively expensive" and time-consuming. The project would have cost $41 billion and taken 25 years to complete - and it would have raised residential electric bills by 125 percent.