  By extending the exterior Schwarzschild spacetime in two opposite directions
with the Kruskal method, we get an extension which has the same T-X spacetime
diagram as has the conventional Kruskal extension, while allowing its regions I
and IV to correspond to different directions of the original spacetime. We
further extend the exterior Schwarzschild spacetime in all directions and get a
4-dimensional form of the Kruskal extension. The new form of extension includes
the conventional one as a part of itself. From the point of view of the
4-dimensional form, region IV of the conventional extension does not belong to
another universe but is a portion of the same exterior Schwarzschild spacetime
that contains region I. The two regions are causally related: particles can
move from one to the other.
