  Prolific electron-positron pair production is possible at laser intensities
approaching 10^{24} W/cm^2 at a wavelength of 1 micron. An analysis of electron
trajectories and interactions at the nodes (B=0) of two counter-propagating,
circularly polarised laser beams shows that a cascade of gamma-rays and pairs
develops. The geometry is generalised qualitatively to linear polarisation and
laser beams incident on a solid target.
