Up to 300k GP and 90k Thieving XP per hour!
-50 Thieving (higher level increases success rate, 93% at level 99)
-Stethoscope in inventory or bank (this can be purchased in rogue's den from Martin Thwait, or on the G.E.)
-Plenty of lobster in bank
Start in the rogue's den with a stethoscope in your inventory. When your health gets low or your inventory is full, this bot will bank for you, withdraw some lobster, and continue cracking. By cracking the wall safes, you will receive a random reward composed of uncut gems and GP. A full inventory can be worth up to 90k. 70 thieving XP is also earned on a successful crack.
NOTE: According to the wiki page, a higher Agility level seems to increase your likelihood of receiving more valuable gems, however this is not confirmed and is not a requirement.