  By performing experiment and model studies on a triangular-lattice dipolar
magnet KBaGd(BO$_3$)$_2$ (KBGB), we find the highly frustrated magnet with a
planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which
originates from the intriguing interplay between dipolar and Heisenberg
interactions. The DSL constitutes an extended regime in the field-temperature
phase diagram, which gets lowered in temperature as field increases and
eventually ends with an unconventional quantum critical point (QCP) at
$B_c\simeq 0.75$~T. Based on dipolar Heisenberg model calculations, we identify
the DSL as a Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1)
symmetry. Due to the tremendous entropy accumulation that can be related to the
strong BKT and quantum fluctuations, unprecedented magnetic cooling effects are
observed in the DSL regime and particularly near the QCP, making KBGB a
superior dipolar coolant to commercial Gd-based refrigerants. We establish the
phase diagram for triangular-lattice dipolar quantum magnets where emergent
symmetry plays an essential role, and provide a basis and opens an avenue for
their applications in sub-Kelvin refrigeration.
