Here is our most recent volunteer Jo, on a day out at the Centre for Alternative Technology. I think she is marvelling at the rammed earth, pise de terre, wall in the info centre there, most impressive. It's made from sub soil, compressed in a mould, in situ. Each layer adds a geological stripe to the wall. The idea is that the sun shines through the ceiling of the building and onto the wall, using its mass as a heat-sink to help maintain a constant internal temperature - like a huge in built storage heater.