  We consider splitting type phase transitions between Calabi-Yau fourfolds.
These transitions generalize previously known types of conifold transitions
between threefolds. Similar to conifold configurations the singular varieties
mediating the transitions between fourfolds connect moduli spaces of different
dimensions, describing ground states in M- and F-theory with different numbers
of massless modes as well as different numbers of cycles to wrap various
p-branes around. The web of Calabi-Yau fourfolds obtained in this way contains
the class of all complete intersection manifolds embedded in products of
ordinary projective spaces, but extends also to weighted configurations. It
follows from this that for some of the fourfold transitions vacua with
vanishing superpotential are connected to ground states with nonzero
superpotential.
