According to the Associated Press, tunnel farming is gaining popularity in the United States. Tunnel Farming is a specialized form of greenhouse farming suitable for growing larger harvests of crops than a conventional greenhouse can provide. The tunnel is created by a frame structure and plastic or glass roofing material. By increasing the temperature inside the structure, 'tunnel farms' also allow crops to be harvested inside the tunnel rather than relocating of seedlings for planting. The 'tunnels' allow farmers to extend the length of their growing season by several weeks, providing for increased revenues. Farmers also note that tunnel farmed crops are of higher quality than their outdoor counter parts. A quick search on the subject shows that farmers of strawberries and tomatoes are currently the largest proponents of the method.
Tunnel farming has several drawbacks at present. One is the increase in relative humidity of the space. The constant high humidity promotes the grow of fungi and diseases of the plants. Temperature can be difficult to control, if the temperatures outside get too high farmers roll up the sides or ends of the structure.
If the trend towards increased use of indoor farming continues, perhaps it will open up a market for improving automated farming techniques or lead to an improved knowledge of the use of enclosed habitats such as the biosphere.