  Resonances in ultracold collisions involving heavy molecules are difficult to
understand, and have proven challenging to detect. Here we report the
observation of magnetically tunable Feshbach resonances in ultracold collisions
between $^{23}$Na$^{40}$K molecules in the rovibrational ground state and
$^{40}$K atoms. We prepare the atoms and molecules in various hyperfine levels
of their ground states and observe the loss of molecules as a function of the
magnetic field. The atom-molecule Feshbach resonances are identified by
observing an enhancement of the loss rate coefficients. We have observed three
resonances at approximately 101 G in various atom-molecule scattering channels,
with the widths being a few hundred milliGauss. The observed atom-molecule
Feshbach resonances at ultralow temperatures probe the three-body potential
energy surface with an unprecedented resolution. Our work will help to improve
the understanding of complicated ultracold collisions, and open up the
possibility of creating ultracold triatomic molecules.
