  A polycrystalline sample of the hyperhoneycomb iridate $\beta$-ZnIrO$_3$ was
synthesized via a topochemical reaction, and its structural, magnetic, and
thermodynamic properties were investigated. The magnetization and heat capacity
data show the absence of long-range magnetic order at least down to 2 K. A
positive Curie-Weiss temperature $\theta_W \sim 45$ K probed by the temperature
dependence of magnetic susceptibility indicates that a Kitaev interaction is
dominant. These observations suggest that a quantum spin liquid may have been
realized. Furthermore, the observation of linear temperature-dependent
contribution to the heat capacity with no magnetic field effect evidences
gapless excitation. These facts are surprisingly contrary to the chemical
disorder evidenced by the crystallographic analysis. By discussing the
differences in the size effect of Z$_2$-fluxes in 2D and 3D Kitaev magnets, we
propose that there is a hidden mechanism to protect the quantum spin liquid
state from chemical disorder.
