The cruise ship Silver Explorer glides quietly through Prince Christian Sound. [Photo by Bruno Cazarini/For China Daily]

We are floating on top of a frozen margarita.

That's how our shipboard naturalist John Fonseca describes the slushy ice that the Silver Explorer is plowing through as we enter Greenland's Prince Christian Sound, named after a long-ago Danish royal who became King Christian VIII. In the distance, the water surface is a smooth sheet of dark-green jade under the overcast sky.

The cruise ship suddenly feels larger as we slip into the narrow passage of the sound. Bigger and bigger chunks of sea ice float by, like huge gobs of whipped cream dropped on the surface by some quirky Norse god. The sculpted forms pass so close, it's tempting to stretch out a finger for a poke and a taste.

At its base, the rocky shore on each side is often green with moss and lichen. But it quickly becomes less hospitable and even forbidding, its glacier-scarred face climbing steeply until disappearing into low, filmy clouds. Some patches of ice have lingered since last winter, though the August sun is converting the most exposed into waterfalls that trickle merrily into the brilliant sea.

Leaning on the observation deck at the ship's Level 6, Fonseca tells us we're looking at rocks that are 2 billion years old. Some of the harsh striations have been dated to the tectonic plate shifts of 65 million years ago that broke up the Earth's landmass into the continents we know today.

There are about 50 passengers on board, most huddled among the rails at 8 am in the bright red parkas provided by the Silversea cruise line. The air temperature is 9 C－the water temperature is-12 C, expedition leader Stefan Kredel announces on a loudspeaker, just in case anybody is planning to strip off for a morning swim.