By far the most perilous route is the Libya-Italy sea crossing, where more than 2,500 people have perished since March. In the worst incident, in late April, a grotesquely overladen fishing trawler capsized and sank within sight of a rescue ship; of the estimated 800 migrants aboard, only 28 were saved.

Largely in response to that tragedy, a handful of rescue vessels — notably those operated by the medical-relief organization Médecins Sans Frontières (M.S.F.) — now patrol the waters off Libya in hopes of intercepting the boats.