You could say the biggest trend in luxury expedition cruises is that, well, it's such a trend.

Between 2016 and 2017 alone, 12 new expedition ships have been ordered -- the majority in the luxury category -- and several existing luxury ships have gone "expedition" via renovation.

Jack Ezon of Ovation Travel, which caters exclusively to high-end travelers, says business for luxury expedition cruses has grown 22 percent in the last three years, a growth he attributes to three main drivers: the dawn of luxury products in the expedition cruise category; shorter itineraries that are attractive to a younger audience and others with limited time off; and climate change, which has created a sense of urgency to see places, before it's too late.

Pairing luxury with adventure at sea was a concept started by the French cruise line Ponant in 1988 -- still considered the leader in the field. But in recent years, as small ship (as in river) cruising has become more popular, adventure travel has also expanded. With well-traveled boomers looking for new experiences and frontiers, the luxury expedition cruise experience has caught on. It's also drawing first-timer cruisers who were never attracted to the traditional cruise concept. Cruising to explore new parts of the world, exploring by Zodiac craft and kayaks, or even helicopters and submarines, then returning -- mud on your boots, exhausted from hiking a glacier or kayaking among seals in the Galapagos -- to a glass of Champagne or a massage at the spa. What sets these cruises apart from other expedition cruises is the five- or six-star service, the high ratio of crew to passengers, the personal service and the lavish amenities, from Veuve-Clicquot Champagne served in Baccarat glasses to bath amenities from purveyors like L'Occitane, Aveda, Hermes, and linens from Matouk and Sferra.

While the competition increases, and more and more cruise lines enter this niche market, the lines can get a little blurry; after all, everyone's got their own definition of luxury -- not to mention adventure. The hallmark of true expedition is all about going deeper into a destination, through activities and opportunities ashore, as well as through expedition leaders -- rather than cruise directors -- onboard.

Here are some of the trends we're seeing in luxury expedition cruising.