A skeleton crew was evacuated from Radio City Music Hall Friday night after a fire broke out in an underground transformer vault nearby, authorities said.

The blaze erupted at about 8:50 p.m. in a vault on West 51^st Street near the historic Midtown venue, which had no show scheduled.

About 50 minutes later — after the four workers on duty had left — there was an explosion that sent a manhole cover flying and could be heard blocks away. No one was injured.