  We present the detailed bifurcation structure and associated flow patterns
near the onset of zero Prandtl number Rayleigh B\'enard convection. We employ
both direct numerical simulation and a low-dimensional model ensuring
qualitative agreement between the two. Various flow patterns originate from a
stationary square observed at a higher Rayleigh number through a series of
bifurcations starting from a pitchfork followed by a Hopf and finally a
homoclinic bifurcation as the Rayleigh number is reduced to the critical value.
Global chaos, intermittency, and crises are observed near the onset.
