  We investigate the confining properties of the QCD vacuum with $N_f=2$
flavors of dynamical quarks, and compare the results with the properties of the
quenched theory. We use non-perturbatively $\mathcal{O}(a)$ improved Wilson
fermions to keep cut-off effects small. We focus on color magnetic monopoles.
Among the quantities we study are the monopole density and the monopole
screening length, the static potential and the profile of the color electric
flux tube. We furthermore derive the low-energy effective monopole action.
Marked differences between the quenched and dynamical vacuum are found.
