The Siebengebirge began as a series of failed volcanoes between 28 and 15 million years ago, late in the Oligocene and into the Miocene. Magma pushed up through the Rhineland crust but mostly never reached the surface. It cooled in place into trachyte, basalt, and andesite, hard volcanic rocks that resisted erosion while the softer sediments around them washed away. What remains today is a cluster of more than forty rounded peaks on the east bank of the Middle Rhine, southeast of Bonn, the highest being the Grosser Olberg at 460 meters. None of them is dramatic by mountain standards. All of them are dramatic by Rhineland standards, because the Rhine valley is otherwise flat, and the Siebengebirge erupts out of it without preamble. The volcanic past also gave the range its quarries. Drachenfels trachyte built Cologne Cathedral. Local basalt paved roads across half of medieval Germany. The hills were a stone source for centuries before they were a tourist destination.

The first and most common theory is the obvious one: sieben means seven, gebirge means hill range, and locals counted seven prominent hills when they coined the name. The fact that the seven they counted shift depending on viewpoint, and that none of the lists exactly matches the seven highest peaks, did not seem to bother anyone. In medieval German, seven was a useful magical number, the size of a dozen with the added bonus of being slightly mysterious, and an unknown territory called Sieben-anything sounded properly impenetrable. The second theory is linguistic and unromantic. The word sieben here is a corruption of siefen, derived from the Middle Low German sipe, meaning a wet hollow or a small trickling stream. The Siebengebirge would then be the wet-hollow mountains, a perfectly accurate description of a range full of springs and small valleys. The third theory is the one historians like best as a story. It traces sieben to sieden, meaning to boil, and points out that medieval Cologne banned soap-boilers - Seifensieder - from working within city limits because the smell was unbearable. The soap-boilers retreated upstream and worked in the hills. Siedengebirge, boiling-mountain, became Siebengebirge over the centuries. Three theories. Pick one. The hills do not seem to mind.

Quarrying ate into the Siebengebirge through the 18th and 19th centuries. The Drachenfels alone was being chiseled away to feed Cologne Cathedral's centuries-long construction, and by the mid-1800s entire flanks of the smaller hills had been turned into open pits. The Prussian government bought out the Drachenfels quarry in 1836, an unusually early piece of conservation thinking, and over the following decades a coalition of local landowners, naturalists, and Rhineland politicians pushed for broader protection. The Drachenfels itself was declared a national protected site in 1956. Today the Naturpark Siebengebirge covers most of the volcanic peaks under environmental protection, and supports a network of waymarked walking trails that draws day-hikers from Bonn, Cologne, the wider Ruhr, and across the border from the Netherlands. The hills that medieval superstition once called sinister and impenetrable have become one of Germany's most-walked recreation forests.

If you take a boat upstream from Bonn and pick out the conventional seven, you start with the Drachenfels (321 m), the most photographed peak in Germany after the Zugspitze. Behind it, slightly inland, sits the Wolkenburg (324 m), once topped by a castle the quarrymen demolished. The Petersberg (331 m) carries the Hotel Petersberg on its terraced summit, the postwar guest house where Konrad Adenauer signed the Petersberg Agreement in 1949. South of those is the Lowenburg (455 m), with the ruins of a 12th-century castle on its peak. The Lohrberg (435 m) sits behind. The Nonnenstromberg (335 m) and the Grosser Olberg (460 m) round out the standard count, though many local lists swap one or another for the Himmerich or the Lohrberg's neighbor. The point is that nobody quite agrees, and the disagreement is part of the place. There are also nearly forty more hills in the range that almost nobody outside the area can name. Trenkeberg. Weilberg. Stenzelberg. Geisberg. Zickelburg. They are part of why the count was always slippery and the name was always negotiable.

The Siebengebirge was where Rhine Romanticism got its altitude. The river flows past castles further south at the Loreley and through gorges further upstream toward Mainz, but the Siebengebirge gave English and German travelers their first proper hills after the flat run upstream from the Netherlands. Lord Byron stopped at the Drachenfels in 1816 and put the rock into Childe Harold's Pilgrimage. Heinrich Heine grew up in Dusseldorf within sight of the range and used it as imaginative geography in his poetry. By the mid-19th century the Siebengebirge was the standard first-day stop on any properly Romantic Rhine tour. Cooks and Murray's guidebooks pushed the same itinerary. The Drachenfelsbahn rack railway opened in 1883 to handle the demand. Today the same hills host the Bonn-area weekend traffic, the Dutch caravan crowd, the school field trips, and the occasional confused American expecting taller mountains. The number is still seven. The hills are still forty. The name still means whatever you decide it means.