Swimming below icebergs

You are swimming close to an iceberg with a convex lower surface. You calculate at what slope you have to swim down so that, whatever the direction in which you swim, you can be sure that you will not collide with the iceberg. This limiting slope is intimately related to the existence of subtangents to the iceberg that satisfy various conditions. These considerations lead to generalizations of Rockafellar's Maximal Monotonicity Theorem, of which we give acomplete new proof. We also discuss related open problems on maximal monotonicity and subdifferentials, and generalizations of recent results on the existence of subtangents separating the epigraphs of proper convex lower semicontinuous functions from nonempty bounded closed convex sets, with some control over their slopes.