Manhole covers have rocketed into the sky all over the state, in cities including Worcester, Somerville, and Boston’s Back Bay.

To understand how the heavy metal circles can get launched into the sky, 7NEWS was given an exclusive look below the street, tagging along with Eversource crews. 7 Investigates discovered chambers packed with the electrical wiring that power businesses, streetlights, and homes above.

“The cable has a conductor where all electricity runs through, and on top of that conductor is insolation that protects that cable from faulting,” said Bill Ritchie, Vice President of Electric Field Operations at Eversource.

Over time, weather and wear can break down that protection, and the lines can short out. When that happens, crews said the air in the space heats and expands,

See Full Page