Tunnels or hoophouses, are temporary agricultural structures with arched or hoop frames, and are covered with clear plastic. They can be easily covered and uncovered, and assembled and disassembled in order to move to a different field site. Most tunnels are passively solar-heated using no electricity. High Tunnels are usually high enough to drive a tractor through. Crops are grown in the ground, usually with drip irrigation. Crop tunnels are relatively inexpensive technology.