What's a walipini? It's a sunken or pit greenhouse that combines passive-solar design with earth-sheltered building to create an environment for year-round growing even in very cold climates. The principle is similar to a coldframe, but a walipini is much deeper and bigger; think a coldframe on steroids.
How does it work? It starts with a deep hole in the ground, below the frost line, so that the space is warmed by thermal mass on all sides. Then a structure is created to capture and store daytime solar radiation, keeping the indoor temperature much warmer than the outdoor temperature. It often includes a "cold sink" beneath raised beds, where the coldest air can settle away from the plants.
Who came up with it? It was developed more than 20 years ago in South America. (The term itself, which translates to "place of warmth," comes from the Aymara Indians of Bolivia and Peru.) Walipinis are now being used in cold climates elsewhere, including Canada and China.