  The search for quantum spin liquids (QSL) -- topological magnets with
fractionalized excitations -- has been a central theme in condensed matter and
materials physics. While theories are no longer in short supply, tracking down
materials has turned out to be remarkably tricky, in large part because of the
difficulty to diagnose experimentally a state with only topological, rather
than conventional, forms of order. Pyrochlore systems have proven particularly
promising, hosting a classical Coulomb phase in the spin ices
Dy/Ho$_2$Ti$_2$O$_7$, with subsequent proposals of candidate QSLs in other
pyrochlores. Connecting experiment with detailed theory exhibiting a robust QSL
has remained a central challenge. Here, focusing on the strongly spin-orbit
coupled effective $S=1/2$ pyrochlore Ce$_2$Zr$_2$O$_7$, we analyse recent
thermodynamic and neutron scattering experiments, to identify a microscopic
effective Hamiltonian through a combination of finite temperature Lanczos,
Monte Carlo and analytical spin dynamics calculations. Its parameter values
suggest a previously unobserved exotic phase, a $\pi$-flux U(1) QSL.
Intriguingly, the octupolar nature of the moments makes them less prone to be
affected by crystal imperfections or magnetic impurities, while also hiding
some otherwise characteristic signatures from neutrons, making this QSL
arguably more stable than its more conventional counterparts.
