Cellars, especially vaulted cellars, are often used for gastronomic operations. This phenomenon occurs worldwide, but there are regions where the principle is a local tradition, so to speak. We only list restaurants that have really been set up in a cellar underground. Like all uses of daily life, eating and drinking in a cellar is rather uncomfortable. For a start, it is rather cool and damp there. Nowadays, of course, cellars can be heated without any problems, but cellar restaurants are more traditional places to eat and drink. However, a reference to alcohol is quite common, perhaps also because a certain alcohol concentration reduces the sensation of cold.
Traditional establishments often grew out of a wine cellar or beer cellar. At first, people came to the cellar to buy wine or beer. Then they might start tasting the wine before buying it and eventually it became a kind of underground pub. If the innkeeper then also sells something to eat, we have a restaurant. But cellars were also often converted into wine bars, taverns or restaurants. If there is no more space left in a building, it might be possible to set up an additional restaurant in an unused cellar.
In Germany, you can find a Ratskeller in many historic town halls. Ratskeller are a result of the fact that town halls often had a communal wine or beer cellar. For storing tithes, as bonded warehouses, or perhaps just for the personal stock of the aldermen. Councillors' cellars are usually traditional inns and offer good home-style cooking. This name also exists in the U.S.A., in areas with many German immigrants.
The term Underground restaurant on the other hand, though a valid translation, typically means something different. Such restaurants are not subterranean, they are part of an underground or illegal network of a supper clubs or closed door restaurants. Operated out of someone's home, they generally bypass local zoning and health-code regulations, and of course the tax. Advertised by word of mouth they are becoming increasingly popular, but if they are not under-ground we do not list them.