Please Explain: Why Is That Manhole Cover Flying At My Head?

New York Magazine explains once and for all what happens when manhole covers explode:

The copper electrical wiring running beneath the streets is hung on the manhole walls and sheathed in insulation, which can crack and warp owing to age (many are 60 years old), chemical corrosion (a major culprit is road salt, which is carried down with rain), or hungry rats.

Cables carry an average of 13,000 volts. With demand up, the cables have to carry more power and begin to heat up. This heat, coupled with the electricity leaking through the cracks in the wiring, starts to burn the insulation.

Carbon monoxide, an extremely flammable gas, is released from the smoldering insulation and collects in the empty chamber; the cover is pushed up like a lid on a pot of boiling water.

An electrical spark can ignite the gas. This is surprisingly common: In one 24-hour period in July, the Fire Department reported 25 manhole explosions in Astoria. Not all result in the covers being shot into the air: That depends on how much gas and electricity is involved. But some covers have been flung over 50 feet.