Cataclysm Timewalking Bonus Event is Underway

Look here, look here

For a few days, adventure back in time to Deathwing’s shattering of Azeroth. Cataclysm Timewalking is available through Monday.
When you queue up for Cataclysm Timewalking, you’ll find yourself heading into:
Grim Batol
The Stonecore
Lost City of Tol’vir
The Vortex Pinnacle
Throne of the Tides
End Time
Vendors in Dalaran, Shattrath, Orgrimmar, and Ironforge are waiting to take your hard earned Timewarped Badges in exchange for all sorts of wondrous rewards such as mounts, transmog gear, and commendations that increase your reputation with Burning Crusade, Northrend, and Cataclysm factions. You should also pick up the event-only quest “A Shattered Path Through Time” in your Garrison. It rewards a Seal of Inevitable Fate and an Iron Fleet Treasure Chest when you complete five Timewalking dungeons.
Which of the six Cataclysm dungeons is the hardest when you’re Timewalking?