  Room-temperature superconductivity has been one of the most challenging
subjects in modern physics. Recent experiments reported that lanthanum hydride
LaH$_{10{\pm}x}$ ($x$$<$1) raises a superconducting transition temperature
$T_{\rm c}$ up to ${\sim}$260 (or 215) K at high pressures around 190 (150)
GPa. Here, based on first-principles calculations, we reveal the existence of
topological Dirac-nodal-line (DNL) states in compressed LaH$_{10}$. Remarkably,
the DNLs protected by the combined inversion and time-reversal symmetry and the
rotation symmetry create a van Hove singularity (vHs) near the Fermi energy,
giving rise to large electronic density of states. Contrasting with other La
hydrides containing cationic La and anionic H atoms, LaH$_{10}$ shows a
peculiar characteristic of electrical charges with anionic La and both cationic
and anionic H species, caused by a strong hybridization of the La $f$ and H $s$
orbitals. We find that a large number of electronic states at the vHs are
strongly coupled to the H-derived high-frequency phonon modes that are induced
via the unusual, intricate bonding network of LaH$_{10}$, thereby yielding a
high $T_{\rm c}$. Our findings not only elucidate the microscopic origin of the
observed high-$T_{\rm c}$ BCS-type superconductivity in LaH$_{10}$, but also
pave the route for achieving room-temperature topological superconductors in
compressed hydrogen-rich compounds.
