Guild Wars 2

Sorry to bother but my group and I have found six of the seven hidden rooms. We cannot seem to find the seventh after two hours. We know that it may be able to be discovered through the usage of a bomb. Even a hint would be great. Thank you all for your time.

Found all of them
Spoilers Here:

1. The first is behind the Waterfall at the spawn
2. The second is in a corner all the way back from the first checkpoint (Facing the Checkpoint)
3. The third is right on top of the drop on the third checkpoint (Use the one tree to the direct right to jump up)
4. The forth is at the corner of the end area
5. The last three are the shops

Once again I apologies if this destroys the Secret Aspect but for some it is difficult.