GW2 Master of Secrets World 2 Achievement can be completed upon finding 17 hidden rooms in World 2 Zone 3.
From the Start to the Checkpoint 1
Hidden Room 1: At the first uphill turn right, on the corner, behind the conifer is a hidden maze. Follow the left wall, to the hidden room entrance, that opens with flute song 332331 that you should learned at world 2. Enter hidden room, climb to the top, and jump down, there is a hidden area with 2 Polar Bears. Bomb a wall next to the 2nd bear and make the entrance to Hidden Room 1.
Hidden Room 2: Bellow the first Polar Bear on the main road, there are 2 bomb-able walls, that open Hidden Room 2 entrance.
Hidden Room 3: There is a cave entrance to shopkeeper room, it is on your main road, you can not miss it at all, since there, you can buy a torch to melt down iced waterfalls.
From the Checkpoint 1 to the Checkpoint 2
Hidden Room 4: Soon after the Checkpoint 1, jump to the left toward the yellow bauble at the bottom. Follow the left wall, and throw a bomb on a wall, in the dark corner. This is Hidden Room 4 entrance.
From the Checkpoint 2 to the Checkpoint 3
Hidden Room 5: On the platform with 2 Bananas and yellow bauble, continue toward the end of platform. In a corner between this platform, and the lower one, there is a bomb-able wall, that reveals Hidden Room 5 entrance. Note: When you enter it, position your camera in a bird perspective, since you will start falling down upon entering, and there is one stone pillar that you should land on.
Hidden Room 6: From this pillar turn right, following the path toward the other side. There is a part of wall that you can bomb, and reveal entrance to Hidden Room 6.
Hidden Room 7: On the large platform with a teleport spot, go to the other side, and make 2 jumps. One on the big pillar, and another on a platform bellow. Wall opens upon flute song 332331.
Hidden Room 8: On the lower level from this area, there is another hidden room, which entrance opens with a help of Flute song 332331.
From the Checkpoint 3 to the Checkpoint 4
Hidden Room 9: From the Checkpoint 3, throw a bomb at the right wall. There is a Hidden Room 8.
From the Checkpoint 4 to the Checkpoint 5
Hidden Room 10: In the middle of the room with ground ice spikes play a flute song 332331, that opens hidden entrance. This leads to a hallway with 6 frozen waterfalls.
Hidden Room 11: On the second to last frozen waterfall, start melting your way to a large room with many Bananas in. Killing them all, opens area to Hidden Room 11, and another Flute Song lesson.
From the Checkpoint 5 to the Checkpoint 6
From the Checkpoint 6 to the Checkpoint 7
Hidden Room 12: This is foggy weather part of zone 3. At this part, you should pass the only frozen waterfall, and head right from there, over a disappearing clouds, to the wall that opens hidden room entrance with a famous song 332331.
Hidden Room 13,14,15: Inside the previous room, there are 3 hidden room entrances, that you reveal upon bombing walls.
From the Checkpoint 7 to the Checkpoint 8
Hidden Room 16: Before a long wooden bridge, and Checkpoint 8, there is another hidden room, which door opens with a Flute song 332331.
From the Checkpoint 8 to the Checkpoint 9
Hidden Room 17: At the edge of the platform with large Yeti on, to the left from the Checkpoint 8, jump toward the only platform across, and once again play the flute song 332331, to enter the last Hidden Room.
From the Checkpoint 9