Some CD versions of Nevermind feature a hidden track called "Endless, Nameless," which begins at the 13:51 mark on track 12 (about 10 minutes after the end of "Something in the Way").
The semi-improvised "noise" song was recorded after a particularly
stressful take of "Lithium". The song was not on the album's first CD
pressing, as the mastering engineer had thought he had reached the end
of the album. For unknown reasons, the song was removed from American
pressings of the disc in 1994 (year of Cobain's death), and all
subsequent CD pressings have lacked the song. Cobain's death may have
inspired the removal of the song. Endless, Nameless was on the UMD version of Nevermind.
I have seen versions with it sell for huge amounts on Ebay. It is considered extremely rare, some people do not even realize they have the hidden track. It was never widely made public. I only know, because at one time I owned it