  Bulk superconductivity in a topological semimetal is a first step towards
realizing topological superconductors, which can host Majorana fermions
allowing us to achieve quantum computing. Here, we report superconductivity and
compensation of electrons and holes in single crystals of the nodal-line
semimetal CaSb$_2$. We characterize the superconducting state and find that
Cooper pairs have moderate-weak coupling, and the superconducting transition in
specific heat down to 0.22 K deviates from that of a BCS superconductor. The
non-saturating magnetoresistance and electron-hole compensation at low
temperature are consistent with density functional theory (DFT) calculations
showing nodal-line features. Furthermore, we observe de Haas-van Alphen (dHvA)
oscillations consistent with a small Fermi surface in the semimetallic state of
CaSb$_2$. Our DFT calculations show that the two electron bands crossing the
Fermi level are associated with Sb1 zig-zag chains, while the hole band is
associated with Sb2 zig-zag chains. The Sb1 zig-zag chains form a distorted
square net, which may relate the $M$Sb$_2$ family to the well known $M$SbTe
square net semimetals. Realization of superconductivity and a compensated
semimetal state in single crystals of CaSb$_2$ establishes the diantimonide
family as a candidate class of materials for achieving topological
superconductivity.
