  We report on the coherent excitation of the ultranarrow
$^{1}\mathrm{S}_0$-$^{3}\mathrm{P}_2$ magnetic quadrupole transition in
$^{88}\mathrm{Sr}$. By confining atoms in a state insensitive optical lattice,
we achieve excitation fractions of 97(1)% and observe linewidths as narrow as
58(1) Hz. With Ramsey spectroscopy, we find coherence times of 14(1) ms, which
can be extended to 266(36) ms using a spin-echo sequence. We determine the
linewidth of the M2 transition to 24(7) $\mu$Hz, confirming longstanding
theoretical predictions. These results establish an additional clock transition
in strontium and pave the way for applications of the metastable
$^{3}\mathrm{P}_2$ state in quantum computing and quantum simulations.
