A world-cruise arms race has begun, with luxury lines competing to make these sailings of 100-plus days ever more enticing. Although they’ve traditionally employed older ships for these lengthy voyages, some companies now use newer, bigger vessels with more amenities. To satisfy a growing demand for longer sailings that include exotic, lesser-known destinations, some are increasing their length as well. They’re also adding more overnights in ports, maiden calls and exclusive shoreside events that give guests especially memorable experiences.

These cruises often sell out fast even though they’re typically put on sale about two years out — with some lines making cabins available three years ahead. Here are five exciting options that offer plenty of reasons to spend up to six months at sea. But note that itineraries may change since they’re planned so far in advance.

40 Countries With Splendor

Regent’s 2027 world cruise is a 140-night voyage aboard the 746-guest Seven Seas Splendor. It’s the largest ship the line has tapped for such purposes and the first time it will use one of its Explorer-class ships, which offer a higher standard of luxury. After departing Miami on January 11, 2027, the Splendor will visit 71 destinations in 40 countries and conclude in New York. It will cover more than 35,000 nautical miles as it sails the Caribbean, the Pacific Islands, Australia, New Zealand, Asia, Africa and Europe, making 14 overnight stays. The itinerary’s 73 UNESCO World Heritage sites include the Elephanta Caves near Mumbai, India, where Hindu temples were carved in the fifth and sixth centuries. In Málaga, Spain, an exclusive Roaring Twenties party will feature a display of haute couture fashion from the 1920s and restored vintage cars. International business-class airfare is a welcome inclusion. Rates from $91,499.