Manholes explode when combustible gases build up in the confined underground space and a spark or electrical arc ignites them. The rapid expansion of hot gas creates enough pressure to launch a cast-iron cover weighing over 250 pounds into the air, sometimes several feet high. Between 2009 and 2018, New York City alone recorded nearly 4,000 manhole explosions, injuring at least 57 people. The problem is persistent, predictable in its causes, and surprisingly difficult to eliminate.
What Builds Up Underground
Beneath city streets, manholes connect networks of electrical cables, sewer lines, and utility tunnels. These enclosed spaces collect gases from two main sources: decomposing organic matter and degrading electrical equipment. Either one can turn a manhole into a sealed chamber filled with fuel, waiting for a spark.
Sewage systems naturally produce methane, a colorless, odorless, and highly flammable gas created when bacteria break down organic waste without oxygen. Even small concentrations of methane in a sealed space can create an explosive atmosphere. Hydrogen sulfide, the gas responsible for a rotten-egg smell, also accumulates from bacterial activity and adds to the hazard. Carbon dioxide collects as well, and while it doesn’t burn, it displaces oxygen and can intensify pressure changes during an explosion. Ammonia, produced when urine decomposes, rounds out the chemical mix in sewer-connected manholes.
The second source is electrical. Most urban manholes house high-voltage power cables insulated with plastic polymers. Over years of use, these cables degrade. Localized hot spots, partial electrical discharges, and small arc faults cause the insulation to break down chemically, a process called pyrolysis. This decomposition produces its own flammable gas byproducts. Arc faults in particular generate large amounts of explosive gases while burning holes through the insulation, creating both the fuel and the potential ignition source in one failing piece of equipment.
How a Spark Triggers an Explosion
Gas alone doesn’t cause an explosion. You need the gas to reach the right concentration and then encounter an ignition source. In electrical manholes, the most common trigger is an arc from a damaged cable. Aging insulation cracks, water seeps in, and electricity jumps across the gap. That arc, which can reach temperatures well above 300°C, ignites whatever flammable gases have accumulated in the space.
Once the gas ignites, the fire rapidly heats the air inside the manhole. Pressure builds in a fraction of a second. A standard heavy-duty manhole cover has an area of roughly half a square meter and weighs about 114 kilograms (around 250 pounds). It sounds immovable, but the math is surprisingly modest: an internal air pressure of just 2,236 pascals, equivalent to a pressure head of about 9 inches of water, is enough to overcome the cover’s weight and send it flying. In a real explosion, pressures can exceed that threshold many times over, which is why covers have been documented launching high enough to damage vehicles, shatter windows, and injure pedestrians.
Why Winter and Wet Weather Make It Worse
Manhole explosions spike during specific weather conditions. Winter brings road salt, which dissolves into water and seeps underground. Saltwater is far more electrically conductive than fresh water, so when it reaches aging cable insulation, it dramatically increases the chance of a short circuit or arc fault. This is why cities like New York see clusters of manhole events during and after winter storms.
Heavy rain creates similar problems. Flooding pushes water into electrical manholes that are designed to stay dry, submerging cables and connections that were never meant to be wet. The combination of moisture, corroded insulation, and high voltage is exactly the recipe for an underground ignition. Summer heat plays a role too, since higher temperatures accelerate the chemical breakdown of cable insulation and increase gas production from organic decomposition in nearby sewer lines.
The Scale of the Problem
New York City’s fire department responded to nearly 4,000 manhole explosions over a ten-year span from 2009 to 2018, an average of roughly 400 per year, or more than one per day. At least 66 people were injured during that period, including 9 more in 2019 alone. As one city official put it, “Once a month or twice a month, we have to turn on the news about an exploding manhole.” And New York is simply the city with the most documented data. Any dense urban area with aging underground electrical infrastructure faces the same risk.
The injuries range from burns and hearing damage to broken bones from being knocked down by the blast. Flying manhole covers are the most dramatic danger, but the superheated gases that vent from the opening can also cause severe burns to anyone standing nearby. Fires sometimes spread to underground cable networks, knocking out power to entire blocks.
Warning Signs Before an Explosion
Manhole explosions aren’t always sudden. Many are preceded by visible or audible clues. Smoke or steam rising from a manhole cover or from cracks in the surrounding pavement is one of the clearest warnings. A humming or buzzing sound can indicate electrical arcing underground. A sulfur or chemical smell near a manhole suggests gas buildup. In some cases, nearby streetlights flicker or dim as failing cables draw irregular current.
Utilities also monitor for stray voltage, which is electrical current that escapes damaged cables and energizes metal surfaces like manhole covers, lampposts, and grates. Any confirmed voltage reading of 1 volt or higher on a street-level structure is classified as hazardous and signals a problem in the underground system. These readings often identify failing infrastructure before it reaches the point of explosion, though testing every manhole regularly in a city with tens of thousands of them is a massive logistical challenge.
What Cities Do to Prevent Them
The most direct engineering fix is vented manhole covers. Traditional solid covers seal the space completely, allowing pressure to build with no outlet. Vented covers have slots or holes that let gas escape gradually, preventing the pressure from ever reaching the threshold needed to blow the cover off. They don’t prevent underground fires, but they dramatically reduce the risk of an explosive launch.
Utilities also run cable replacement programs, prioritizing the oldest sections of underground wiring where insulation is most degraded. In New York, the utility Con Edison has spent billions replacing aging infrastructure, but the sheer scale of the underground network means the work takes decades. Some cities use gas sensors installed inside manholes that alert crews when combustible gas concentrations rise to dangerous levels, allowing them to ventilate the space before ignition can occur.
Routine inspection programs target manholes for signs of water intrusion, cable damage, and gas accumulation. But with hundreds of thousands of manholes in a major city, inspections are inherently reactive. Most manhole explosions still happen without advance warning to the public, which is why the simplest safety advice remains straightforward: if you see smoke, smell something unusual, or hear buzzing coming from a manhole, move away from it quickly and report it.