I built a house two years ago that has a large portion of it actually inside a hill. One small room that is “below grade” was originally designated to be a wine cellar, and I’ve noticed that winter or summer it is always cool (and sometimes cold) in there. It is a 10 foot by 11 foot windowless room with 6" concrete walls all around, above and below it. We put a mixture of hardwood flooring and slate over the concrete floor to make it more attractive.
It tends to stay about 65 +/- degrees most of the year and has no ventilation other than a single door. It probably gets as warm as 70 degrees in the summer, and as low as 50 degrees in the winter. There is nothing in the room currently to control either temperature or humidity, but I plan to put something in the room to monitor these.
I don’t knoiw what the humidity is exactly, but I live in California near the Pacific Ocean so our humidity is usually between 30% and 60% most of the year. 
Would this cellar be considered within the bounds of a normal wine cellar’s environmental spec’s or should I add some type of climate control? This house, when it sells, should be worth about $2M so I don’t want to cut corners.