  We report experimental studies of disorder-induced transitions between
quantum-Hall, metallic, and insulating states in a very dilute two-dimensional
electron system in silicon at a magnetic field corresponding to Landau level
filling factor $\nu=1$. At low disorder, the lowest extended state at $\nu=1$
is below the Fermi energy so that the system is in the quantum Hall state. Our
data show that with increasing disorder (but at constant electron density and
magnetic field), the extended state DOES NOT DISAPPEAR BUT FLOATS UP IN ENERGY
SO THAT THE SYSTEM BECOMES INSULATING. As the extended state crosses the Fermi
energy, the conductivity $\sigma_{xx}\sim e^2/2h$ has temperature dependence
characteristic of a metallic system.
