  The structures of the ground and excited states of 12Be were studied with
antisymmetrized molecular dynamics. The ground state was found to be a state
with a developed 2-alpha core with two neutrons occupying the intruder orbits.
The energy levels of the newly measured spin-assigned states were described
well, except for the $1^-_1$ state. The calculations indicated that many exotic
cluster structures appear in the low-energy region. The widths concerning alpha
and 6He decays were discussed by using reduced width amplitudes.
