  We report a study of quantum oscillations in the high-field
magneto-resistance of the nodal-line semimetal HfSiS. In the presence of a
magnetic field up to 31 T parallel to the c-axis, we observe quantum
oscillations originating both from orbits of individual electron and hole
pockets, and from magnetic breakdown between these pockets. In particular, we
find an oscillation associated with a breakdown orbit enclosing one electron
and one hole pocket in the form of a `figure of eight'. This observation
represents an experimental confirmation of the momentum space analog of Klein
tunneling. When the c-axis and the magnetic field are misaligned with respect
to one another, this oscillation rapidly decreases in intensity. Finally, we
extract the cyclotron masses from the temperature dependence of the
oscillations, and find that the mass of the 'figure of eight' orbit corresponds
to the sum of the individual pockets, consistent with theoretical predictions
for Klein tunneling in topological semimetals.
