Hopes you don't fear darkness, 'In the dark' is the first cave without light, wich means... no map for you, only screenshots :[
First, mine the suspicious stones below you and follow the passage. Mine the 3 soft stone blocks at its end and watch out for the purple blob that is waiting for you behind it. Open the door at your left and take the bronze key. Keep following the passage, dig the dirts that block your way and open the bronze door below it, take the coal and the silver key.
Follow the passage, be carefull about the purple rat, dig the dirts but don't release the boulder, open the silver door and kill the blue blob behind it. Take the gold key, mine the soft stone from its right and let the boulder fall. Go up, don't dig the gravels, keep go up and dig the dirts and open the treasure chest.
Now that you got the treasure you can go back and dig the gravels, kill the red bat behind it and found the blue chest. You'll need a blue key and a hammer to clear that mine.