When a utility worker in Key Largo, Fla., noticed that a section of a paved street was not settling properly, he decided to remove a manhole cover and descend into the earth.

Moments later on Monday morning, the 15-foot-deep hole went silent. Sensing the man was trapped, a fellow utility worker climbed into the drainage hole to rescue him.

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When he, too, stopped responding, a third worker entered the same hole. All three men died, overcome by poisonous fumes underground, the Monroe County Sheriff’s Office said in a statement.

A Key Largo firefighter who made a desperate attempt to save the men also became unconscious within seconds. The firefighter, Leonardo Moreno, an eight-year veteran of the …