Growing technique for winter gardening I heard about?

Someone told me that you can dig a hole in the ground and put something like a cold frame over the top of it to grow vegetables like kale and peas in the winter. I can't find any info on this. My guess is the hole is to provide extra room for the plant to grow.

Actually, it's for protection from cold. If you google "pit greenhouse" you'll see a wealth of examples. Pit greenhouses are an old technology for avoiding the fuel costs of heated greenhouses.

http://www.backwoodshome.com/articles2/martin104.html

horse manure hotbeds! if you have any horse owners around you, they will PAY YOU to clean out their stalls of this stuff and put it in a pile. if you offer to haul it off, some will pay you MORE!

Depending on where you live and what you want to grow, you may not need the heat provided by the manure. This winter, I am experimenting with growing lettuce in earthboxes, with the earthboxes enclosed in hay bales which are topped with used double-pane windows. Hay bales will start to steam and release heat, and I am betting that in my climate, this will keep the lettuce in good condition and even growing. We'll see!

I dug a pit that I'm going to use for a toilet in the future, and a tomato plant self seeded in it in spring, and is still flowering and fruiting almost into December, while my other tomato plants all stopped setting fruit. I never watered the plant and I'm in the Mediterranean, with no summer rain. So pits have certainly piqued my interest.