There is nothing more abundantly available and earth-friendly to build with than the earth beneath your feet! Some of the longest lasting structures are earthen; these include adobe, cob, and rammed earth buildings. Each of these techniques needs a particular mix of clay and sand to form solid walls. The ideal ratio of clay to sand is somewhere between 1:4 and 1:2. Other fibers, such as straw, might be thrown into the mix for added strength. Additionally a stabilizer can be added to make it more weather resistant.
A more modern twist on using earth for building is with earthbags. Traditional burlap bags can be used, but polypropylene bags are stronger and don't rot (if kept out of the sun.) Earthbags can be stacked vertically to make either straight or curved walls, or they can be stacked into dome shapes that require no additional framing to make a roof. The material that the bags are filled with can have a wide range of textures or composition. My house is made with bags filled with crushed volcanic rock, which is quite insulating. Other people have built with adobe-filled bags, shell-filled bags, or sand-filled bags. It is important to experiment with the particular soil you want to use to make sure that is becomes sufficiently consolidated that the shape is stable. It is possible to add a stabilizer, such as Portland cement, to some loose soils that would otherwise not be stabile enough.