Neighborhoods across the country are falling dangerously dark overnight as cities struggle with a staggering increase in the theft of copper wire from utility poles and streetlights.

The thieves are reselling the increasingly valuable copper wire for cash.

“This is a really challenging thing to deal with because it’s not a traditional form of crime that most people understand,” said Ben Stickle, a former police officer and a criminologist at Middle Tennessee State University specializing in metal thefts.

Mr. Stickle said copper theft is a growing national problem fueled by the black market, and most police departments are unsure how to tackle the crime.