  We develop methods for constructing explicit generators, modulo torsion, of
the K_3-groups of imaginary quadratic number fields. These methods are based on
either tessellations of hyperbolic 3-space or on direct calculations in
suitable pre-Bloch groups, and lead to the very first proven examples of
explicit generators, modulo torsion, of any infinite K_3-group of a number
field. As part of this approach, we make several improvements to the theory of
Bloch groups for K_3 of any field, predict the precise power of 2 that should
occur in the Lichtenbaum conjecture at -1 and prove that the latter prediction
is valid for all abelian number fields.
