Myst, at more than 3.5 million units sold, is the world's most popular PC entertainment title, and after four years is still riding the top of the charts. This fact remains, despite a significant number of players who never get past the game's first tough puzzle: the rotating lighthouse.

For whatever reasons that Myst was such a success, Riven has a lot of the same. You'll see the same click/move/slide-show interface, along with similar mechanical and navigational puzzles and the now familiar wander and wonder gameplay. Riven's markedly improved graphics and sound further enhance the ambiance and make Riven even more immersive than its predecessor. The puzzles and problems are more cohesive, and there is more of a storyline, but in the end, Riven is only an evolutionary improvement over Myst.

You begin where Myst left off. Atrus (played once again by Rand Miller), the father of the two feuding sons in Myst, has another task for you: Rescue his wife from the evil clutches of Atrus's father Gehn, then permanently imprison Gehn in one of those "trap" books from Myst.

Your quest takes place in Riven, a fanciful world created by Gehn in the tradition of his ancestors - by simply imagining it and writing about it. Riven (from the ancient English word "rive," which means "to rip apart") is a collection of small islands connected with catwalks, aerial trams, and a minisubmarine. You will expend much of your effort just figuring out how to get around. In many instances you'll find a switch in an obscure location, then take a circuitous route to the gate it opens.

Unlike Myst, these islands have human inhabitants, although you'll be hard-pressed to actually see any. There are plenty of spherical, earthen huts and occasional fleeting human encounters, but there is no interaction or dialogue. The oblique storyline is played out in the islands' objects and the implications you can draw from them, such as a disturbing kid's toy, a Wizard of Oz-like throne, and a frog trap. And if these are too obtuse you can always arduously pore through the voluminous journals you'll acquire.

Riven's two major puzzles require solving several miniproblems that in turn require close observation of Riven's denizens, colors, and unique numbering system. Gathering those elements becomes routine but just as you get comfortable with the concept there is a clever extra challenge tossed in - a missing color, a broken device, an obtusely revealed animal. Much of Riven's prerelease PR emphasized the environment as a source of clues and puzzle solutions. It is, but not to the large extent and subtle degree you may expect.

There are no technological breakthroughs but plenty of high-quality production values. The QuickTime animations - doors opening, levers moving, drawbridges dropping - are larger and blend well with the backgrounds, but that's principally a reflection of improvements in that display method. The sound effects are perfect and worth the price of high-end speakers, but there is no support of newer 3D audio technology. The graphics are superbly, even fanatically, detailed but that's more an aesthetic decision than a technical achievement.

For a game that has low-tech system demands, it consumes an inordinate amount of hard drive space - 140MB. Plus, if you play it for an extended period of time, you may encounter audio lapses. I missed a critical end-game aural clue, but after rebooting, and replaying that scene, it played just fine. Riven ships on five discs, but you can start saved games with only one of them and must click slowly through four animations before you can load a game .

With a walk-through in hand, Riven took only about four hours. Without it, I'd still be at it, doing a lot of wandering, mapping, and recording of my observations. And just like Myst, that's what Riven is all about: exploring a new world full of unique beings, organic machines, and family intrigue. There is no inventory, no need to combine mysterious potions, and no board game puzzles with little connection to Riven's world. Rather, it's a leisurely paced, all-encompassing, mentally challenging experience. If you enjoyed Myst, you'll thoroughly enjoy Riven.