Looking at photos of natural houses is a fun way to learn about natural building. Almost half the homes in the world are made with earth (adobe, cob, rammed earth, earthbag, etc.), so there are countless photos of houses made like this on the Internet. Try searching for photos in your favorite search engine using the search terms: earth houses + the name of country you are interested in. Google Images is my current favorite (although the quality is slipping lately). Try various search terms to see what works best. You could try searching: earth dome or earth dome home or earth roundhouse or earth sheltered house. You get the idea. In foreign countries you could search similar words in the local language.