It's a dangerous mine, and it's not a secret. You keep mining until you find a ladder, grab it, and go down to the next level. The point of this is find the ladder in each level the fastest you can before the next earthquake occur. On the first few weeks, I remember me, my friend Hanson, PigParty, Ryjofa, Ywr, and a bunch of random people mined through the cave and reached the final level, which is the 5th level. It's pretty hard to get there nowaday because no one even mine in the dangerous mine anymore. Forgot to mention, minerals are random, and if you're lucky, you'll receive some rocks, or even high-level gems like emeralds and diamonds.