  We employ angle resolved photoemission spectroscopy (ARPES) to investigate
the Fermi surface of RuO$_2$. We find a network of two Dirac nodal lines (DNL)
as previously predicted in theory, where the valence- and conduction bands
touch along continuous lines in momentum space. In addition, we find evidence
for a third DNL close to the Fermi level which appears robust despite the
presence of significant spin orbit coupling. We demonstrate that the third DNL
gives rise to a topologically trivial flat-band surface state (FBSS) at the
(110) surface. This FBSS can be tuned by surface doping and presents an
interesting playground for the study of surface chemistry and exotic
correlation phenomena.
