Germany's largest spring really spurts out water – in damp times up to 24,000 litres of water shoot out on the surface. Like from a powerful high temperature boiler, it bubbles and gurgles round the clock.
At the other end of the spring surrounded by a low stone wall, the Aach creek snakes its way romantically between birch trees and willows along the mill wheels and past geese houses through the Swabian little towns of Beuren and Singen and then after 20 kilometres flows into Lake Zeller and Lake Constance, which after a long journey via the Rhine drain into the North Sea.
You would suspect hordes of day-trippers here, where three countries border – Germany, Austria and Switzerland. All the more because the source of the Aach has been hiding a dark secret for thousands of years. But a signpost next to the "Jägermühle" pub on the river bank and an empty carpark are the only traces of tourist activity in Aach this Saturday.
200 metres down the road, in the guesthouse "Zur Krone" there's a similar effect: 280 shillings per night with an alibi breakfast and WC in the corridor. The owner of the "Krone", Walter Villing, knows more about the digging going on in search of an underground river above the Aach spring, but there is no trace of overflowing business acumen: "Yes, it wouldn't be bad if they found something", he thinks and whilst on the subject added: "If you write something in the newspapers, then say at the same time that I'm looking for a replacement to take over my guesthouse...
In the "Jägermühle" Harald Schetter is waiting with photos of caves he took himself. He is actually a broker by profession, but in his free time he prefers to goby around with flippers and a compressed-air tank in underwater caves. Schetter has been diving since 1962, yet what began as a hobby to compensate for his "red-tape" job in Yugoslavia quickly became a passion, when the now 56-year-old moved to the Aach region with his wife Myra: "In the meantime I have got to know every hole here. Whether it has water in or not!"
What has it got to do with the mysterious disappearance of the Danube, of which very little is known in Austria? "Already under Maria Theresia there was a suspected link between the seepage of the Danube at Immendingen and Fridingen (see graphic) and the unusually powerful Aach-spring 14 kilometres south", explains Harald Schetter. "In this area, the Danube carried almost no water even at that time, and in summer and autumn you could walk on the river bed."
According to the town chronicle, a certain Professor Knop proved for the first time in 1876 the existence of the underground river course of the Danube: under the watchful eyes of the local tax authorities he tipped 200 hundredweight of cooking salt into the seeping crevices of the Danube. 20 hours later the spring water in Aach tasted salty!
Under sometimes life-threatening conditions, adventurers have been repeatedly trying since 1886 to dive against the strong current through the Aach-gorge into the underground course of the river, in order to explore the cave system on the Danube. Some of these attempts ended fatally.
In the 60s the "Large hall of lakes" was discovered 120 metres away from the Quelltopf (see map). Harald Schetter has in the meantime risked the treacherous dive there countless times and, together with his partner Axel Gnädinger, explored and photographed every corner. Both found, amongst other things, the bones of a 5000 year-old cow and traces of bat droppings, which prove that the caves were once open.
Schetter describes the underwater route to the lakes in a report: "You could only see a few metres ahead. The current was so strong that I needed several attempts to get through the "nozzle" (Note: A narrow spot at the cave entrance). With my stomach to the ground I forced myself forward along lumps of rock, kicked as hard as I could with my flippers, grasped at loose rubble and pulled myself along centimetre by centimetre. Thick clouds of clay were stirred up. Every metre of this blind journey was a risk. I shuddered at the thought of having to dive back through this earthy broth just by holding the guide wire. But the tension eased as soon as I saw the air bubbles bursting on the surface of the water. When I surfaced, a terrific sight greeted me. My searchlight lit up a gigantic cavern. Steep rising walls, bizarre rock formations and a large lake, which removes for a moment the darkness that has reigned for many thousands of years and presents itself as an unreal, green, glittering surface..."
In the meantime we established that on the side of the Aach, the river course abruptly ended after 600 metres in the direction of the Danube at a cave further along (see map). At some point an area of the roof above the underground river measuring about 100 metres caved in. Since then, water from the Danube has seeped through countless crevices in the porous cliff. Schetter: "It is only from the magnitude of the huge sinkholes in the forest that you can tell how the water must have created such a gigantic cave system over the years."
Right in that spot, upstream of this land subsidence, a handful of of hobby geologists are relentlessly digging for the underlying treasure trove of caves that are the "lost Danube", which, as the adventurers hope, should stretch for 12 kilometres downstream to the point where the Danube infiltrates.
Like the pointed arches of a gothic cathedral, beech trees and birch trees arch 30 metres above the sinkhole shafts from all sides into the starry evening sky. Through a crack in the clay shines a spotlight.
Smeared with mud from his Wellington boots to his full red beard Lothar Dietrich, a bookkeeper for "Deutsche Telekom", appears at the top of a ladder that leads directly down a hole a good 30 metres deep into the ground: "Hi, I'm Lothar. And you're probably the visitor from Vienna", he says and laughing, holds a soaking wet, waxy glove out to greet me.
More clay-covered figures with hard-hats and head-lamps surface from the shaft one after another like a family of meercats: "That's Frank and Stefan, a policeman and a computer scientist", presents Lothar. "This here is Oliver, who has a day job at a solar cell company and that is Niko, the vice-president of our club."
The members of "Friends of the Aach caves" meet here every Friday night for a few hours of idyllic hours of hard knuckle work, by day tex advisers, lawyers, doctors, an engine driver and even a priest. Even the descent into the real digging spot 70 metres down is an adventure, and is only recommended with a helmet, lamp and waterproof rubber overalls – if at all. Even a statics expert has turned away shaking his head: "perilous!"
Over steel spears, wobbly ladders and a one-metre-high, steep exit you climb and crawl down next to a plastic pipe that transports grit and sand into the depths to cement out the brittle marl stone. Water drips everywhere from the walls and small mushrooms grow on the old wooden joists which used to support the roof.
"A few years ago we discovered a cave under here", shouts Lothar during his descent behind me. "We call it the 'grey hall', our entrepot on the way to the Danube." Computer technician Stefan is already shovelling sand into a concrete mixer which, taken apart up above, was put back together down here. The finished cement is then taken down deeper using several self-built hand winches. Loose rubble is brought back up in replacement.
Six hours later the night shift has ended and the new roof, beneath which further digging is to take place over the next week, is finished. Moaning and hunched over we crawl back to the surface one after the other. Breathe deeply and emerge out of the muddy tarpaulin. Over a beer together in the site hut on the forest trail, the group of men are motivated to carry on: The tunnel diggers dream of "sailing a raft on the underground Danube" and "unspoiled dripstone caves with glistening calcium crystals on the walls."
"It can only be a question of a few more metres", reflects Harald Schetter looking at his sketches in the "Jägermühle" the next day. But he said that at the very beginning of the dig ten years ago.