At EA's Game Changers event yesterday, Maxis senior VP Lucy Bradshaw confirmed that SimCity is back , and showed off a very pretty, but very gameplay-less, trailer. Today at GDC, however, Maxis went into detail when it unveiled the game's engine, GlassBox, which I also got a look at during a recent visit to the studio. Wow. Spore was certainly ambitious, and The Sims has reached incredible commercial success, but GlassBox may turn out to be Maxis' most impressive achievement yet.

The previous SimCity games rely on relatively high-level statistics to tell us what's going on--the pollution number goes up, so the happiness number goes down. GlassBox does the opposite. It simulates the little things--thousands of individual Sims--and lets the city mechanics emerge naturally. You won't have to look at a spreadsheet or graph to identify a crisis, because you can watch it all happen in real-time. Pollution will taint the soil and thicken the atmosphere with smog, Sims will get sick and fill the hospitals, businesses will lose employees, and everyone will be really ticked off about the whole thing.

Fire stations no longer provide statistical coverage--a fire truck must drive from the station to the scene of a fire, and the longer it takes, the longer the building burns. Every car and every pedestrian (which are referred to as "agents") is someone going somewhere to do something, and every traffic jam is the natural result of the patterns they create as they navigate your roads.

Creative Director Ocean Quigley demonstrated how agents operate by artificially populating a closed loop of road with vehicles and pedestrians.

"We've created all these people in here, and there's no jobs for them, no houses for them, and no place to shop. They're basically all miserable and would love to get out of here as fast as possible," said Quigley, as hundreds of his sadsack residents drove and trudged in an endless circle.

"So let's connect their little maze to the outside world, and these people are going to abandon the city as fast as they can. So, all the little agents, they have an agenda, a mood, something they want to do. They hate it here, they want to get out, and so that's what they're doing."

Now consider that Maxis is striving to simulate tens of thousands of agents at a time, and that resources such as power, water, coal, and oil are also treated as individual units, as well as every house, business, and factory, and you can start to get an idea of the incredible number of emergent possibilities GlassBox introduces.

Of course, if SimCity were an ASCII game, this wouldn't be quite as impressive. Lots of games feature detailed and intricate simulations. But it's not--every one of these agents is modeled, animated and dynamically lit. The engine's data is even detailed enough to play a different sound for a sedan turning a corner than it does for a truck approaching a stop light.

And, most importantly, everything you see and hear in GlassBox is really something happening in the simulation--it never creates ambiance for the sake of it. The jingle of a shop's door is a Sim entering, and the sound of a cash register is a purchase being made. A power plant's smoke puffs represent pollution entering the atmosphere, and the size of its coal pile represents its current supplies. When I saw all of this (and much more), happening at once, I really got the feeling that I was looking down at a tiny living world. So, wow . I think a bit of excitement for the project is justified, even at this early-ish stage of development.

But, of course, this is still an early stage (SimCity's release date is projected for sometime in 2013, and Maxis is only releasing concept art so far), and things can change. And while GlassBox is the foundation, there are all kinds of rules and systems that are going on top of it to turn it into a proper SimCity game, so there's still a lot to discover. We've got 8-pages of details in the May issue of PC Gamer US, which is on its way to subscribers, and will be on newsstands later this month. We'll keep you updated as development continues and more of our questions are answered.