  In insulators, the longitudinal resistivity becomes infinitely large at zero
temperature. For classic insulators, the Hall conductivity becomes zero at the
same time. However, there are special systems, such as two-dimensional quantum
Hall isolators, in which a more complex scenario is observed at high magnetic
fields. Here, we report experimental evidence for a quasi-quantized Hall
insulator in the quantum limit of the three-dimensional semimetal SrSi2. Our
measurements reveal a magnetic field-range, in which the longitudinal
resistivity diverges with decreasing temperature, while the Hall conductivity
approaches a quasi-quantized value that is given only by the conductance
quantum and the Fermi wave vector in the field-direction. The quasi-quantized
Hall insulator appears in a magnetic-field induced insulating ground state of
three-dimensional materials and is deeply rooted in quantum Hall physics.
