Fire in the hole, manhole covers in the air
Large cities require a lot of subterranean infrastructure, such as plumbing, transportation and electrical lines. Most of the times these systems work well enough, and the city functions without serious interruption. But under the right conditions, component breakdown can mean more than a blackout— it can actually cause the ejection of manhole covers, launching the 50-pound hunk of metal straight up into the street above.
Subterranean launch conditions
These launches are due to the slow decay of electrical components below the street. There is a lot of wiring underground to account for the demanded capacity plus redundant wiring to keep electricity flowing when the first wire fails. The wire that fails will burn out, like a fuse, and in older wires that burn will release flammable gas into the wiring duct. Once there is enough gas built up, the next spark can ignite all of it, causing an explosion big enough to launch the manhole cover into the air, like a really dangerous release valve.
These problems are exacerbated in the winter, as the salt laid down on the road is carried into the wiring ducts by melting water. The brine then accelerates the rotting of the wires’ plastic insulation and acts as a conductor for easier discharge of electricity.
Safer manholes and sewers
Fortunately, the older wiring is slowly being replaced, so blown wires won’t release volatile gas. Work is also being done at the Electric Power Research Institute to better understand the causes of these explosions as well as develop launch-proof manhole covers. These covers allow pressure to be relieved from under the street, but still stay latched to the ground so they don’t become a threat to everyone around them.
Source: Why Cold Cities Have More Exploding Manhole Covers by Carl Engelking, D-brief