You will need a few tools to be able to find this cache. I will provide you with one of those tools at GC584A. You will need to take great care and follow the instructions to the letter.
You will need to take a flashlight with you, yep it is dark in there.An old pair of pants and a sense of adventure would also help.
When you get to the first stage a big green guy on the wall will point you to the ninth step. Here you will find another tool. Be sure to return it as you found it. This is the last tool you will need. It will be time to gather up your courage and get down to the hunt. After this you are on your own, there is no way I am going back in there.