  The fate of integer quantum Hall effect (IQHE) at weak magnetic field is
studied numerically in the presence of {\it correlated} disorders. We find a
systematic {\it float-up} and {\it merging} picture for extended levels on the
low-energy side which results in direct transitions from higher-plateau IQHE
states to the insulator. Such direct transitions are controlled by a quantum
critical point with a {\it universal} scaling form of conductance. The phase
diagram is in good agreement with recent experiments. The issue of continuum
vs. lattice model is also discussed.
