[Following three previous articles - for '20 Unusual Control Schemes', '20 Open World Games', and '20 Difficult Games', respectively, Gamasutra's 'Game Design Essentials' series looks at the design lessons from titles in which 'the player must solve mysteries' - from finding secrets to wrestling with algorithmically generated content.]

Not really all that very long ago, a game was released called The Legend of Zelda. For the time it was ground-breaking, but as often happens with games, many of the really interesting things about it were not copied in the inevitable onslaught of games it inspired in the following years.

One of those features was Zelda's huge variety of secret passages. While other games, including later Zeldas, will sometimes throw in a poorly-hidden secret chamber once in a while, the first Zelda did it all the time. Nearly half of its overworld screens, in fact, contain a special area of some type, offering rewards ranging from handfuls of rupees to entire dungeons that must be found to win the game.

The existence of so many things hidden in the game that don't have to be found lends the game a certain quality, one best described as verisimilitude. Verisimilitude is a useful word to use in describing video games. It means the quality of being like life, but the connotations are more profound than petty "realism," which has been redefined in the game review sphere as the quality of the graphics. Properly used, the word means that there seems like there is a world outside the borders of the screen, happening regardless of what the player does. It implies the existence of a fully-fleshed world, one that's more than a mere collection of polygons or tiles that might as well be sealed in Plexiglas. It allows a game to better enable the player to forget that it is, really, just a game.

Of course in games this is usually faked, although with varying degrees of success. Grand Theft Auto and Dead Rising, where one can tell the story to take a running jump and go off and explore on one's own, fake it pretty well. Games that force a strong, linear narrative on the player, not so well.

Not all games require that the player be immersed, and pretend to actually be a participant in the game world. Indeed, it's probable that too many games desire this state, and fail badly when they try. But there are other reasons for wanting to instill mysterious elements into a game. But I should define my terms. By saying "mysterious," I mean games that hide some aspect of their play or workings from the player. These are games in which the unraveling of hidden knowledge is essential to the game. These are games in which the player must solve mysteries -- thus the word "mysterious."

There are two major ways to do this. The most common is to include a lot of pre-made content the player must discover. This is, by far, the most common method, and nearly all games do this in one way or another. First-person shooters only reveal the portion of their territory which is visible to the player's camera. Even simple games like Pac-Man could be termed mysterious, in that the behavior of the monsters is algorithmic, but difficult to figure out during a game. All games are mysterious if the category is defined too broadly, so we will concern ourselves with games that go to unusual lengths, games that take the matter deeper than just the unveiling of new content:

The standard disclaimer: This is not intended to be a "top 20" list, and these games are not presented in a ranked order. Some of the games may seem to only be peripherally associated with the topic. That's because, to truly understand some concept, it's useful to look at both obvious and non-obvious cases. Most of these games are older, but this is as intended. Older is not the same as worse! The purpose of the article is to show how other games have done it over the years, and hopefully to inspire you in working on your own projects. You could even play them to see the principle in action.