I am thinking of digging a six feet narrow hole, and filling it in with a piece 2ft wide piece of sewage pipe, and using that as barrel style root cellar. The reason I want to go so deep, is that I am under the impression that the deeper it is, the cooler it will get.
Is this true? How much cooler can I make it?
No, it is not true that necessarily the deeper you get the cooler it gets.
For really deep holes it is actually the opposite, the deeper you get the warmer the temperature gets. This is called the Geothermal Gradient. This states that temperature goes up 25C per 1KM of depth.
For the first couple of meters the temperature will likely drop or raise (depending on the above ground temp) dramatically. But it will then level off and begin to slightly increase. This is because your moving away from the influence of the Sun and start to be influenced by the temperature of the earth's core.
For a root cellar, you're not going to go 1km deep (obviously). You really need to think of a root cellar as a way to maintain (roughly) a constant temperature. The depth (that your likely to dig) alters this slightly but not much. This temperature could be hotter or colder than the temperature above ground, as shown in the picture below which are taken from http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm.
This figure shows the maximum temperature deviation from the annual average is dampened out at about 30 feet depth. You can refer to the figure´s source for more detailed information, i.e. on seasonal changes.
For any reasonable depth (ie. something you'd be willing to dig without specialized machinery), a deeper hole makes for a more stable temperature. The extra mass of soil surrounding your cellar acts to average out temperature changes: shallow burial averages out day-night shifts, while deeper averages out seasonal changes as well. The end result is that a deep enough hole will hold a constant temperature, at the annual average temperature of your location.
