  Ultrastable high-spectral-purity lasers have served as the cornerstone behind
optical atomic clocks, quantum measurements, precision optical-microwave
generation, high resolution optical spectroscopy and sensing. Hertz-level
lasers stabilized to high finesse Fabry-P\'erot mirror cavities are typically
used for these studies but are large and fragile such that they have remained
laboratory instruments. There is a clear demand in rugged miniaturized lasers
operating potentially at comparable stabilities to those bulk lasers. Over the
past decade, ultrahigh-Q optical whispering-gallery-mode (WGM) resonators have
served as a platform for low-noise microlasers but have not yet reached the
ultimate stabilities defined by their fundamental noise. Here, we show the
noise characteristics of WGM resonators and demonstrate a resonator-stabilized
laser at the fundamental limit by compensating the intrinsic thermal expansion
of a WGM resonator, allowing a sub-25 Hz linewidth and a 32 Hz Allan deviation
on the 191 THz carrier in 100 ms integration. We also reveal the environmental
sensitivities of the resonator at the thermodynamical noise limit and long-term
frequency drifts governed by random-walk-noise statistics.
