This circular room is the entrance way to the Department of Mysteries. The entire room is black, from the shiny black marble floor to the handleless black doors set around it. Blue candles burn in the room. When the entrance door is closed, the circular wall rotates, making it difficult to tell which door leads where.
Hermione used the Flagrate spell to mark the doors they'd opened, which allowed the members of Dumbledore's Army to find the door to the Hall of Prophecy.