The Eifel is a hill country, which has a complex geology resulting in a nice and varied landscape. But there are no important karst areas with spectacular sight. There is also no important mining activity.
One part of the Eifel, called Kalkeifel (limestone Eifel) is karstified and has a lot of karst springs. Those springs deposit limestone in form of tufa or travertine, which often contains small primary tufa caves.
The most impressive geologic sight of the Eifel are the so-called Eifelmaare or Maare, almost circular lakes between 10 an a few hundred meters in diameter. Often they are located in circular depressions. This are the remains of the local volcanism, the craters of eruptions which are now water filled. The volcanism is based on an intrusion of lave, which is deep below. When ground water met the hot lava, it evaporated immediately causing a steam explosion. Most of those volcanoes erupted only once, and the amount of lava which was produced is very low. However, the volcanism has not ended, although there has been no eruptions for a very long time. There are springs of volcanic carbon dioxide in the area, which is produced by lava.