A manhole exploded near 113 East 39th Street around 4:25 a.m. Five buildings were evacuated, but no injuries were reported.
The FDNY found high carbon monoxide levels near the building, and residents may be out of their homes for many hours. It's unclear what caused the explosion, but they are not uncommon. Of course, this incident took place just blocks from where a steam pipe exploded last year, killing one woman, critically injuring two people, and injuring many more.
Check out the How Stuff Works explanation on exploding manholes. Slotted manhole covers are considered safer (they "allow the gas to be released less violently, and also give an early warning to possible explosions"); in DC, utility Pepco started replacing old manhole covers with slotted ones.