  Four-dimensional supergravity theories are reinterpreted in a 12-dimensional
F-theory framework. The O(8) symmetry of N=8 supergravity is related to a
reduction of F-theory on T_8, with the seventy scalars formally associated, by
O(8) triality, to a fully compactified four-form A_4. For the N=1 type I model
recently obtained from the type IIB string on the Z orbifold, we identify the
K\"ahler manifold of the untwisted scalars in the unoriented closed sector with
the generalized Siegel upper-half plane Sp(8,R)/(SU(4) \times U(1)). The SU(4)
factor reflects the holonomy group of Calabi-Yau fourfolds.
