  We investigate the structure of center vortices in maximal center gauge of
SU(2) lattice gauge theory at zero and finite temperature. In center projection
the vortices (called P-vortices) form connected two dimensional surfaces on the
dual four-dimensional lattice. At zero temperature we find, in agreement with
the area law behaviour of Wilson loops, that most of the P-vortex plaquettes
are parts of a single huge vortex. Small P-vortices, and short-range
fluctuations of the large vortex surface, do not contribute to the string
tension. All of the huge vortices detected in several thousand field
configurations turn out to be unorientable. We determine the Euler
characteristic of these surfaces and find that they have a very irregular
structure with many handles. At finite temperature P-vortices exist also in the
deconfined phase. They form cylindric objects which extend in time direction.
After removal of unimportant short range fluctuations they consist only of
space-space plaquettes, which is in accordance with the perimeter law behaviour
of timelike Wilson loops, and the area law behaviour of spatial Wilson loops in
this phase.
