  We propose a twelve-dimensional supergravity action, which describes low
energy dynamics of F-theory. Dimensional reduction leads the theory to
eleven-dimensional, IIA, and IIB supergravities. Self-duality of the four-form
field in IIB supergravity is understood. It is necessary to abandon
twelve-dimensional Poincare symmetry by making one dimension compact, which is
to be decompactified in some region of parameter space, such that the physical
degrees of freedom are the same as those of eleven-dimensional supergravity.
This makes T-duality explicit as a relation between different compactification
schemes.
