  We consider Sen's effective action for unstable D-branes, and study its
classical dynamics exactly. In the true vacuum, the Hamiltonian dynamics
remains well-defined despite a vanishing action, and is that of massive
relativistic string fluid of freely moving electric flux lines. The
energy(tension) density equals the flux density in the local co-moving frame.
Furthermore, a finite dual Lagrangian exists and is related to the
Nielsen-Olesen field theory of ``dual'' strings, supplemented by a crucial
constraint. We conclude with discussion on the endpoint of tachyon
condensation.
