Cusco is a departament of Peru where Machu Picchu is located.
The Simpson family visited the site looking for Bart after he had attached a tracking device from his cell phone to a bird, which flew to Machu Picchu. While there Marge was visited by an old Incan god, who told her that she had been overprotecting Bart. Marge realized that he was right and, after finding the bird, she and the family returned back home.
When they returned home, Maggie was accidentally left behind in Machu Picchu and ended up being worshipped as a goddess.