  We model spacetime foam by a gas of virtual wormholes. Then applying an
external field one may change the density of virtual wormholes and try to
organize a wormhole-like structure in space. The relation between an additional
distribution of virtual wormholes and the external field is considered for the
homogeneous case. We show that the external fields suppress the density of
virtual wormholes which sets an obstruction for creating an actual wormhole in
a straightforward fashion. We also present a rough idea of a more complicated
model for the artificial creation of a wormhole.
