Secret levels are levels you usually can't access by normal means, meaning they don’t have a direct passage leading to them. It is theorized there are still many more secret levels to be found out there.
The Basement
The basement was formerly theorized to exist at the bottom floor of the backrooms, but there's no evidence that there is a bottom. Due to this, the former level -1 has been renamed to "The Basement" after another level was named -1. To access this level you have to break and go through the roof on any floor. This level is the most dangerous of all, yet highly skilled people still come and go through this level. The exits to this level are random; the same as the paths to them. People mainly come here to try and find an unknown base.