  Topological matter is known to exhibit unconventional surface states and
anomalous transport owing to unusual bulk electronic topology. In this study,
we use photoemission spectroscopy and quantum transport to elucidate the
topology of the room temperature magnet Co$_2$MnGa. We observe sharp bulk Weyl
fermion line dispersions indicative of nontrivial topological invariants
present in the magnetic phase. On the surface of the magnet, we observe
electronic wave functions that take the form of drumheads, enabling us to
directly visualize the crucial components of the bulk-boundary topological
correspondence. By considering the Berry curvature field associated with the
observed topological Weyl fermion lines, we quantitatively account for the
giant anomalous Hall response observed in our samples. Our experimental results
suggest a rich interplay of strongly correlated electrons and topology in this
quantum magnet.
