Treyarch has been boasting that there are actually three games in Call of Duty: Black Ops 2-- single-player, multiplayer, and Zombies. Though the developer has discussed the first two games in detail, the third leg of the Blops 2 tripod has been kept under wraps--at least, until now. Along with the worldwide reveal trailer of the new Zombies mode, we’ve gotten details on what we can expect from the robust co-op mode, and it’s looking like it’ll be the best undead slaying mode yet. Want to know why? Have a look…

Tranzit is the brand new open world in Black Ops 2 Zombies. A post-apocalyptic, zombie-infested world is free for you to explore, and it’s full of secrets for you to wander off and discover. Up to four players can join together and literally walk from one point of interest to another. Zombies has also stepped out of the Cold War and WWII eras and will have players encountering buildings, locations, and enemies you’d expect to see in a modern-era zombie apocalypse.

You’ll hunker down and fend off undead waves in gas stations, diners, farmhouses, and bus stations. You won’t see undead Nazi soldiers stumbling around anymore--most undead look like once-normal (now infected) citizens wearing hospital gowns and civilian clothing, but Treyarch has also said we can expect new special zombie types as well. Having an open world means, if you end up being overrun by the zombie horde, you’ll have a way out. You can run, on-foot, to a safer location, or take a more efficient mode of transportation…

Need a lift to the diner down the road? Just hop on the bus in Tranzit mode and it’ll drop you off at key locations around the map. But riding the bus isn’t a guarantee that you’ll make it to your destination in one piece; it could actually end up being the more dangerous choice between walking and driving. Zombies can jump onto and attack your bus, ripping off boarded-up windows to get to the juicy, gun-toting flesh bags inside. You’ll be closed in tight, with nowhere to run if a zed breaks through your defenses. Defending this moving zombie beacon seems like it could be a massive undertaking, and a thrilling one at that.

We’ve mentioned before that in Tranzit, you’ll be able to go off the beaten path and discover secrets hidden around the world. Treyarch has told us that some of those secrets will be Easter eggs that pertain to the Zombies lore, but players will also be able to come across “buildable” items. Spread across the Tranzit world are pieces of items that can be put together and used to open new areas. The developers haven’t gone into too much detail on what these pieces and “tools” are, but they seem like they’ll be the primary motivation for venturing off into the frightening world on-foot.

The third mode in Zombies is an eight-player mode called Grief. It pits one team of players against another group of humans, all of whom are desperately trying to survive against the zombies. Both teams will not be able to directly attack the opposing team, but they will be able to affect the survivability of their human opponents.

Redirecting undead hordes or possibly disabling the defenses of the opposition will be the path toward victory. The details on exactly how Grief will work are still scarce at the moment, but we’ll fill you in as soon as we get the official word.

Even though the developers have made a lot of changes to Zombies, those who’ve been enjoying the round-based survival mode need not worry: Survival will be returning. You’ll be able to band together with three other co-op zombie slayers, as you try to hold off wave after wave of undead attackers.

All of the traditional elements will be there. You’ll have to barricade windows and doors, seek out the mystery box for a chance to get a new weapon, grab perks from vending machines, and upgrade your weapons in the Pack-a-Punch weapon transformer. It’s all there, just as you’d expect from Survival. But this time, you’ll be facing the shambling corpses in post-apocalyptic sectioned-off portions of the Tranzit world map.

Just because everything has been revamped and expanded, doesn’t mean the old zombie mode hasn’t received some love and attention. For example, when you and your co-op buddies die, you won’t have to start all the way back at round one. You can choose which round you want to start on--being rounds five, ten, fifteen, and twenty--and receive cash values relative to the starting round, so you’ll be able to purchase the necessary upgrades to survive.

Zombies has also been integrated into the multiplayer engine, meaning that you’ll enjoy the same matchmaking, leaderboards, and stat-tracking features you get on the competitive side of Black Ops 2.

If, for whatever masochistic reason, you want to make Zombies more difficult, you can activate the “Headshots only” challenge, or turn off the magic items, which include the mystery box, perks, and Pack-a-Punch machines. It all goes a long way towards upping the flesh-eating ante.

Treyarch wasn’t able to go into more specific detail with the customization options, but we’re definitely looking forward to how in-depth we can go into adjusting our zombie-blasting excursions.

There you have it. The first details on the upcoming Call of Duty Black Ops 2 Zombies mode have been revealed. What do you think of the new changes? What modes are you most excited to play? Let us know what you think in the comments below.

For even more on Zombies, be sure to check out our What we want from Black Ops 2's Zombies and find everything you need to know on Call of Duty Black Ops 2 on our game page.