  The low-lying energy spectrum of the static-colour-source-anti-source system
in a vacuum containing light and strange quarks is computed using lattice QCD
for a range of different light quark masses. The resulting levels are described
using a simple model Hamiltonian and the parameters in this model are
extrapolated to the physical light-quark masses. In this framework, the QCD
string tension is found to be $\sqrt{\sigma}=445(3)_{\rm stat}(6)_{\rm sys}$
MeV.
