Zynga, which makes hugely popular Facebook “social games” such as Farmville and Mafia Wars, has released details of Adventure World, an ambitious and potentially more absorbing new type of social game.

Adventure World is an Indiana Jones-style strategy game in which players search for historical artifacts and piece them together to earn virtual gold and experience points.

The game is much larger and more complex than anything Zynga has built previously, and calls for players to strategize about not only what to do in the game itself but also which Facebook friends to invite along for the ride. It introduces concepts more commonly found in PC video games, providing a large, complex virtual world. Zynga hopes this will attract new players and keep everyone engaged longer, which is key to its profits.

Zynga’s existing games can be played in as little as a few spare minutes; the company makes money mainly by selling things for players to use inside its games. The company sells a lot of these virtual items; its filing for an initial public offering reveals that Zynga had $91 million in net income last year on $597 million in revenue. That marked a five-fold increase in revenue from the year before.

If Zynga is to keep growing, it needs to develop new kinds of social games, and Adventure World is a bold new step. It is 40 times bigger than the largest version of Farmville, featuring five major environments, including a jungle and the inside of a volcano. It has 35 different maps to explore, puzzles to solve, and enemies to battle on the way to collecting precious artifacts. Unlike other games that run within Facebook, Adventure World is designed to be played full-screen, immersing the player in the world of the game.

Adventure World was built by a team of developers with experience building “massively multiplayer online games” for companies such as Turbine, which makes The Lord of the Rings Online. The game’s creators adapted concepts from this kind of PC game in an effort to provide a richer experience. “We wanted to bring more story to Facebook,” says principal game designer Jesse Kurlancheek.

“There’s the emergence of a middle ground” between casual Facebook games and more immersive games, says David Bisceglia, CEO and cofounder of game company The Tap Lab, which makes location-based social games. He says a number of companies are looking at ways to bring the power of massively multiplayer online games—known for entrancing gamers for years on end—to social networks and mobile devices.

The result is a much more in-depth experience than the average social game. However, Bisceglia notes that developers must still try to preserve the casual feeling. Players need to be able to stop and take a phone call, for example, without losing their place. The key, says Bisceglia, is to balance “synchronous and asynchronous” aspects of the game, allowing players to participate whether or not their friends are playing at the same time.

Adventure World incorporates new technology that Zynga developed called the Boston Rendering Optimization engine (so called because it was created at Zynga Boston, formerly Conduit Labs). This engine allows a complex game environment to run using the browser technology Flash, speeding up the loading and processing of interactive objects and freeing designers to build larger, more complex worlds. Without the engine, players would likely experience slow loading times or hiccups during the game.

Zynga Boston CTO Craig Lancaster says, “We wanted a social game with the style and scope of games we’d played outside of that genre, and we had to build the engine that let us do that.” The company plans to use the new engine to power future games.

Like other Facebook games, Adventure World is designed for short bursts of play. Players might work on a map for 15 minutes during a coffee break, says Kurlancheek. And Zynga is sticking with its proven business model, offering for sale virtual items that will help players complete tasks within the game more efficiently.

In addition, Kurlancheek says, Adventure World’s creators aimed to deepen the social experience. Many of Zynga’s games reward players who recruit friends to help them accomplish in-game tasks. In Adventure World, friends can make different contributions to one another’s progress. A player can gather skills and items and then share them with friends—in order to, say, win renown as the player who does best with the grappling hook.

Zynga has already taught many casual players what it means to level up. Kurlancheek hopes that Adventure World will also tempt casual players to talk about things like getting a party together to explore a new corner of the game. “This is the first time my Mom actually understands what I do,” he says.