  The layered triangular lattice owing to $1:2$ order of $B$ and $B'$ sites in
the triple perovskite $A_3 B B'_2$O$_9$ family provides an enticing domain for
exploring the complex phenomena of quantum spin liquids (QSLs). We report a
comprehensive investigation of the ground state properties of
Sr$_3$CuTa$_2$O$_9$ that belongs to the above family, by employing
magnetization, specific heat, and muon spin relaxation ($\mu$SR) experiments
down to the lowest temperature of 0.1~K. Analysis of the magnetic
susceptibility indicates that the spin-lattice is a nearly isotropic $S = 1/2$
triangular lattice. We illustrate the observation of a gapless QSL, in which
conventional spin ordering or freezing effects are absent, even at temperatures
more than two orders of magnitude smaller than the exchange energy ($J_{\rm
CW}/k_{\rm B} \simeq -5.04$~K). Magnetic specific heat in zero-field follows a
power law, $C_{\rm m} \sim T^\eta$, below 1.2~K with $\eta \approx 2/3$, which
is consistent with a theoretical proposal of the presence of spinon Fermi
surface. Below 1.2~K, the $\mu$SR relaxation rate shows no temperature
dependence, suggesting persistent spin dynamics as expected for a QSL state.
Delving deeper, we also analyze longitudinal field $\mu$SR spectra revealing
strong dynamical correlations in the spin-disordered ground state. All of these
highlight the characteristics of spin entanglement in the QSL state.
