God's Thumb is the top of a mountain found near Camp Green Lake. When there was a lake, God's Thumb was on the other side. This is the place where Caveman (Stanley Yelnats IV) and Zero (Hector Zeroni) took refuge after fleeing Camp Green Lake. The location was given this name after Stanley's Great Grandfather Stanley Yelnats I took refuge on the thumb for 16 days after being robbed by Kissin' Kate Barlow (Katherine Barlow), and was left stranded in the desert. He found his way to God's Thumb, climbed to the base of the rock outcropping, and found the natural spring where Sam had his onion field.
When Stanley Yelnats IV escaped Camp Green Lake to find Zero, they made their way to God's thumb. Stanley and Zero ate sweet onions that were still growing by the spring - the onions fixing the illness caused by Hector Zeroni's 100-year-old Sploosh - and drank the water from the spring, as Stanley sang the pig lullaby. That broke the curse, which allowed the Yelnats family to regain their luck. This place is also known as Sam's Onion Field.