Anand sacrificed a pawn in a sharp line known to both players. White gets dangerous changes on the kingside for the material. Aronian isn't the kind to defend passively and he came up with an ingenious idea to lift a rook and counter-sac a knight to get at the exposed white king. I've often called Anand the world's best tactical defender, but just like he did in his loss to Aronian in a wild Marshall kingside assault in Linares last year, Anand blundered when it looked like his defense was going to hold. Here the culprit was 33.Rg3, which allowed Black a remarkable rook maneuver that left the remaining White forces in chaos. The liquidation required to break the pressure left far too many black pawns for White to handle. As complicated as the position is, Anand couldn't have missed anything deep. The position after 35..Rg4 is lost. 33.Rxd4 looks best for White, though he still has unpleasant moments to come. If 33..Rg4 34.Ne3 Rxh4 35.Nxg2 Rh2 36.Rxd7 Bc8 37.Rc7 Rxg2 and with the bishop pair and Black having only two pawns for the piece White should even be better. 33.Rxd4 Ne5 looks very menacing but the nullifying move 34.Rd8 keeps White in control.