  Recent theoretical studies have extended the Berry phase framework to account
for higher electric multipole moments, quadrupole and octupole topological
phases have been proposed. Although the two-dimensional quantized quadrupole
insulators have been demonstrated experimentally, octupole topological phases
have not previously been observed experimentally. Here we report on the
experimental realization of classical analog of octupole topological insulator
in the electric circuit system. Three-dimensional topolectrical circuits for
realizing such topological phases are constructed experimentally. We observe
octupole topological states protected by the topology of the bulk, which are
localized at the corners. Our results provide conclusive evidence of a form of
robustness against disorder and deformation, which is characteristic of
octupole topological insulators. Our study opens a new route toward
higher-order topological phenomena in three-dimensions and paves the way for
employing topolectrical circuitry to study complex topological phenomena.
