The Rodderberg is an extinct volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 in the east of the municipality of Wachtberg

Wachtberg

Wachtberg is a municipality in the Rhein-Sieg district, of North Rhine-Westphalia, Germany. It is situated approximately 15 km south of Bonn. In 2005 the Wachtberg municipality had approximately 20,000 inhabitants....

 near Bonn

Bonn

Bonn is the 19th largest city in Germany. Located in the Cologne/Bonn Region, about 25 kilometres south of Cologne on the river Rhine in the State of North Rhine-Westphalia, it was the capital of West Germany from 1949 to 1990 and the official seat of government of united Germany from 1990 to 1999....

, Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

. The last eruption was 250,000 years ago.

It is situated just above the Rhine-valley. Together with the Drachenfels

Drachenfels

Drachenfels is the name ofMountains:* Drachenfels , a mountain on the Rhine between Königswinter and Bad Honnef, Nordrhein-Westfalen...

 directly opposite, the Rodderberg narrows the Rhine-valley. This gorge portion is the southern limit of the lowland-bay of Cologne.

The Rodderberg offers views to the Siebengebirge

Siebengebirge

The Siebengebirge is a German range of hills to the East of the Rhine, southeast of Bonn, consisting of more than 40 mountains and hills. It is located in the municipalities of Bad Honnef and Königswinter. It is of volcanic origin and came into being between 28 and 15 million years ago...

, the Rhine-valley and up to the Cologne Cathedral

Cologne Cathedral

Cologne Cathedral is a Roman Catholic church in Cologne, Germany. It is the seat of the Archbishop of Cologne and the administration of the Archdiocese of Cologne. It is renowned monument of German Catholicism and Gothic architecture and is a World Heritage Site...

, about 50 km away.

Geology

The Rodderberg detonated in a phreatomagmatic eruption

Phreatomagmatic eruption

Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and magma. They are different from magmatic and phreatic eruptions. The products of phreatomagmatic eruptions contain juvenile clasts, unlike phreatic eruptions, and are the result of...

 about 250,000 years ago. This happened when the magmatic plume reached the groundwater level of the Rhine (about 50m higher than today). After the explosion, the crater was filled with water that formed a maar

Maar

A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma. A maar characteristically fills with water to form a relatively shallow crater lake. The name comes from the local Moselle...

of about 90 m depth. In the last 250,000 years this feature filled with dust and soil until it was filled up to today's level.

At present, all that is left of the old Volcano is just a small hill which has a shallow craterous form on its surface - if one doesn't know that it is a volcano, you would not see it.