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

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... uced to the critical value. Global chaos, intermittency, and crises are observed near the onset.