  Collisional resonances are an important tool which has been used to modify
interactions in ultracold gases, for realizing novel Hamiltonians in quantum
simulations, for creating molecules from atomic gases and for controlling
chemical reactions. So far, such resonances have been observed for atom-atom
collisions, atom-molecule collisions and collisions between Feshbach molecules
which are very weakly bound. Whether such resonances exist for ultracold ground
state molecules has been debated due to the possibly high density of states
and/or rapid decay of the resonant complex. Here we report a very pronounced
and narrow (25 mG) Feshbach resonance in collisions between two ground state
NaLi molecules. This molecular Feshbach resonance has two special
characteristics. First, the collisional loss rate is enhanced by more than two
orders of magnitude above the background loss rate which is saturated at the
$p$-wave universal value, due to strong chemical reactivity. Second, the
resonance is located at a magnetic field where two open channels become nearly
degenerate. This implies the intermediate complex predominantly decays to the
second open channel. We describe the resonant loss feature using a model with
coupled modes which is analogous to a Fabry-P\'erot cavity. Our observations
prove the existence of long-lived coherent intermediate complexes even in
systems without reaction barriers and open up the possibility of coherent
control of chemical reactions.
