  Preparation of molecular quantum gas promises novel applications including
quantum control of chemical reactions, precision measurements, quantum
simulation and quantum information processing. Experimental preparation of
colder and denser molecular samples, however, is frequently hindered by fast
inelastic collisions that heat and deplete the population. Here we report the
formation of two-dimensional Bose-Einstein condensates (BECs) of spinning
$g-$wave molecules by inducing pairing interactions in an atomic condensate.
The trap geometry and the low temperature of the molecules help reducing
inelastic loss to ensure thermal equilibrium. We determine the molecular
scattering length to be $+220(30)$~Bohr and investigate the unpairing dynamics
in the strong coupling regime. Our work confirms the long-sought transition
between atomic and molecular condensates, the bosonic analog of the BEC-BCS
(Bardeen-Cooper-Schieffer superfluid) crossover in a Fermi gas.
