You can find it by going into the Kalphite Nursery, then taking the tunnel to the east, then entering the room that you fell into during the quest (go all the way east to enter the room). There is a crack in the floor you can mine with level 80 Mining (can be boosted), which will open a fissure and grant 20,000 Mining experience (using bonus experience if available). Enter it and there will be the mask and also a sceptre called Was. Wearing the Scabaras mask together with the Ankh while in the desert is one of the elite Desert Tasks.