Talk about being on top of the world. A three- or four-week cruise (say, between New York and Anchorage, or Greenland to Russia) traverses the sounds, straits, and seas that link the Atlantic and Pacific oceans, with dramatic icy landscapes and visits to Inuit villages. This is a route only the most intrepid explorers of the past could even dream of taking, with Roald Amundsen and his team being the first to successfully navigate it in a trip that took three years. Now, more than 100 years later, luxury expedition ships will take you on this journey. Crystal Serenity will make the trip in August, with planned shore excursions that include kayaking around icebergs and camping on an ice sheet in Greenland. These once-in-a-lifetime journeys have also been offered by Ponant, Silversea, Lindblad Expeditions, and Quark Expeditions.