Can anyone tell me how to get down there? have a slayer task in there.

bring a rope..if you know where it is just use a rope on the tunnel entrance

Its location is west when entering the desert, one single waterskin should do for the trip. You will find a well and you need to use a rope on it then use the well.

 

 

 

If you have started fairy tales part 2 you can use the fairy rings and there's a fairy ring right next to it (still need rope)

 

 

 

If you have completed the quests needed to get into the cave goblin city head further north, in the cave under the agility arena theres a tunnel that leads directly to the kalphite tunnel entrance (not in front of the queen :P)

 

Theres also a fairy ring in that cave