Fertile Ground attached to a basic land you control.
Steps
1Activate the basic land with Fertile Ground attached by tapping it, adding at least .
2Fertile Ground triggers, adding .
3Activate Earthcraft by tapping Horseshoe Crab, untapping the basic land.
4Activate Horseshoe Crab by paying , untapping it.
5Repeat.
6If the basic land is an Island, you may add mana of any color using Fertile Ground's trigger, and you may infinitely repeat steps 3-4 with any other basic land you control.