The beta for Marvel’s Avengers is now available for PlayStation 4 players who preordered the game, and it’s pretty expansive–it covers an early portion of the game’s single-player story, includes a bunch of multiplayer missions, and gives you a chance to take on the roles of four different Avengers. But it also has some secrets mixed in, including a hard-to-find hidden mission that represents the best take on Avengers’ multiplayer so far.The hidden mission is a War Zone set in the Snowy Tundra area of Siberia, which you’ll visit during the story portion of the beta when you play as Kamala Khan and the Incredible Hulk. In that mission, your job is to find a secret SHIELD bunker that’s been hidden away from the enemy forces of AIM. After you finish that mission, Snowy Tundra is no longer accessible from your War Table map where you select missions. But you can return to the location for a new War Zone, provided you find a hidden SHIELD cache in another mission.When you return to the Snowy Tundra, you’ll go after a secret SHIELD vault that’s full of high-powered gear and resources. The difference is, this SHIELD vault is enormous, and requires…