  We report the observation of a three-body recombination resonance in an
ultracold gas of cesium atoms at a very large negative value of the $s$-wave
scattering length. The resonance is identified as the second triatomic Efimov
resonance, which corresponds to the situation where the first excited Efimov
state appears at the threshold of three free atoms. This observation, together
with a finite-temperature analysis and the known first resonance, allows the
most accurate demonstration to date of the discrete scaling behavior at the
heart of Efimov physics. For the system of three identical bosons, we obtain a
scaling factor of $21.0(1.3)$, close to the ideal value of $22.7$.
