We have a half-in-ground cellar, built about 25 years ago. It has a cinder block exterior, with a small layer of insulation between it and the interior plywood. This summer when we got into the mid 90s, the interior temp got close to 80. I have a temp sensor in there so I can monitor it here in the house.
In the winter, it does a better job, it usually maintains a temp about 20 degrees above the outside temp. So, when it’s forecast to drop below 15 or so, we swap out the light bulb with a heat lamp, to keep things from freezing inside.
We keep food in there that we’ve canned ourselves, as well as canned food from the store and some other items, like tea, coffee, beans, etc stored in jars or sealed up in Tupperware type containers. Not the ideal environment for things in the summer, but great in the cooler months. Plus, we don’t have room for all that stuff in our home. We don’t store any veggies in there, other than pehaps some potatoes or onions later in the growing season.
We’ve considered building a new cellar, deeper into the ground, but have other priorities at the moment.