  The Beckenstein-Hawking black hole entropy in string theory and its
extensions, as expressed in terms of charges that correspond to central
extensions of the supersymmetry algebra, has more symmetries than U-duality. It
is invariant under transformations of the charges, involving a 12th (or 13th)
``dimension''. This is an indication that the secret theory behind string
theory has a superalgebra involving Lorentz non-scalar extensions (that are not
strictly central), as suggested in S-theory, and which could be hidden in M- or
F- theories. It is suggested that the idea of spacetime is broader than usual,
and that a larger ``spacetime" is partially present in black holes.
