  The seminal works of Bekenstein and Hawking have revealed that black holes
have a well-defined thermodynamic description. In particular, it is often
stated in the physical literature that black holes, like mundane physical
systems, obey the first law of thermodynamics: $\Delta S=\Delta
E/T_{\text{BH}}$, where $T_{\text{BH}}$ is the Bekenstein-Hawking temperature
of the black hole. In the present work we test the regime of validity of the
thermodynamic description of gravity. In particular, we provide compelling
evidence that, due to quantum effects, the first law of thermodynamics breaks
down in the low-temperature regime $T_{\text{BH}}\times r_{\text{H}}\lesssim
({{\hbar}/{r_{\text{H}}}})^2$ of near-extremal black holes (here $r_{\text{H}}$
is the radius of the black-hole horizon).
