The problem of separation is to find an affine hyperplane, or ``cut'', that lies between the origin O and a given closed convex set Q in a Euclidean space. We focus on cuts which are deep for the Euclidean distance, and facet-defining. The existence of a unique deepest cut is shown and cases when it is decomposable as a combination of facet-defining cuts are characterized using the reverse polar set. When Q is a split polyhedron, a new description of the reverse polar is given. A theoretical successive projections algorithm is proposed that could be used to compute deep facet-defining split cuts.