  We propose a class of nodal line semimetals that host an eight-fold
degenerate double Dirac nodal line (DDNL) with negligible spin-orbit coupling.
We find only 5 of the 230 space groups host the DDNL. The DDNL can be
considered as a combination of two Dirac nodal lines, and has a trivial Berry
phase. This leads to two possible but completely different surface states,
namely, a torus surface state covering the whole surface Brillouin zone and no
surface state at all. Based on first-principles calculations, we predict that
the hydrogen storage material LiBH is an ideal DDNL semimetal, where the line
resides at Fermi level, is relatively flat in energy, and exhibits a large
linear energy range. Interestingly, both the two novel surface states of DDNL
can be realized in LiBH. Further, we predict that with a magnetic field
parallel to DDNL, the Landau levels of DDNL are doubly degenerate due to
Kramers-like degeneracy and have a doubly degenerate zero-mode.
