The lamp has copper and zinc strips coming out from the bottom of it and uses the soil as an electrolyte to power the LED light.
參考:http://chinese.engadget.com/2008/11/17/soild-lamp-burns-leds-and/
參考:http://craziestgadgets.com/2008/04/30/soil-lamp-runs-on-mud/

Soil Lamp 土壤燈