orecastOn the night of 2 January 2000, a street in Bochum-Wattenscheid gave way. By morning three garages and their cars had vanished, and in their place gaped a funnel about 40 metres deep. Nobody had been digging there, and yet this hole was made by people. Four years later the ground opened in Siegen, and in 2010 in Schmalkalden. Our new episode explains why two of these holes have a history on paper and one does not. Then we zoom into Siegen, the city where our data know an unusual amount of old mining.
Two holes with a hundred-year fuse
Emilstraße in the district of Höntrop is a small cul-de-sac with garages. When the ground gave way there without warning, two craters opened with a combined area of about 500 square metres. The Wattenscheid crater is considered one of the largest mining collapses in the Ruhr and made national news for weeks. The cause went back to 1905, when the colliery Vereinigte Maria Anna Steinbank closed and its shaft number 4 was filled in. Ninety-five years later that very fill gave way. Nobody was hurt. There is more in our post on the second world beneath the Ruhr.
In February 2004 it was Siegen's turn. On the Rosterberg, in Gläserstraße, a hole about ten metres across opened in the middle of a residential area, and a full-grown birch disappeared into it completely. Almost a hundred people had to leave their homes, and nobody died. The cause was the Hohe Grethe mine, recorded since the 17th century, with chambers up to eight and a half metres in size. It had closed in 1900. The state paid around four million euros for emergency aid and for making the ground safe. A 2014 report by LWL archaeology on mining at the Rosterberg has a title that could be the motto of the whole episode: "Vergessen bedeutet nicht verschwunden", or forgotten does not mean gone.
Schmalkalden and the second kind of hole
The third case does not fit the pattern. On 1 November 2010, at around three in the morning, a crater opened in a residential area of Schmalkalden in Thuringia. The hole grew to about 30 metres across and about twelve metres deep, swallowing a car, part of a garage and a piece of garden. 26 people had to leave their homes, and nobody was injured. This hole had nothing to do with mining. The ground beneath it contains sulphate rock, which slowly dissolves in water until the roof of a natural cavity can no longer hold. About 40,000 cubic metres of gravel, costing around 270,000 euros, were needed to fill it, and ten years later the ground there was still sinking.
So there are two kinds of hole. A mine only stays alive as long as miners renew the supports and pump out the water. After the last shift the timber props rot, the mine water rises, roofs cave in, and the void creeps upwards decade after decade, like an air bubble rising through honey. When it finally reaches daylight, German miners call it a Tagesbruch, and above ground there is often no warning at all. A natural sinkhole like the one in Schmalkalden is much rarer, but hardly predictable, and even the best map cannot warn you of it. Mining is different. Wherever people dug, they almost always wrote down exactly where. Many of these places are recorded today in open data sources, and the orecast map brings them together.
110 points in a single city
At the time of the episode, our map held almost 26,000 mining points with a hazard marker. On a map of Germany they trace the Ruhr, the Saarland, the Harz and the Ore Mountains, and the Siegerland glows surprisingly densely. This is not a count of every old mine in Germany, because the data vary in density from region to region and also include points from neighbouring countries.
Within the city limits of Siegen alone, our data list 110 adits, shafts and pits, and every one of these points carries a hazard marker. 77 of them have a name, and some sound as if they came from an old storybook, such as Philipps Hoffnung, Grüner Löwe, Silberner Nagel or Vereinigtes Bergmannsglück. They are not 110 separate mines, though, but individual points such as entrances, shafts and parts of workings. The Eisenzecher Zug, for example, appears three times. Our data also show 44 caves and 32 geotopes within the city. You can see what that looks like on Siegen on the orecast map.
Celts, silver and a queen
Why here of all places? The answer goes back a good two thousand years. On the southern edge of Siegen, in Niederschelden, archaeologists excavated a complete iron smelting workshop at the Gerhardsseifen site. It dates from the Middle to Late La Tène period, roughly between 300 BC and the turn of the era. It includes a roasting pit, two dome-shaped bloomery furnaces, a slag heap and a smithing area, and the German Mining Museum in Bochum describes its state of preservation as internationally unique. These Celtic furnaces are considered the largest bloomery furnaces in Central Europe. Archaeological experiments in 2018 and 2019 showed that they produced steel. The furnace scenes follow the excavation findings. Like the scenes from the old mines, they are labelled AI reconstructions in the video, not real footage.
In 1298 Adolf of Nassau granted the Ratzenscheid, a silver district just outside today's city, to his cousins. It is the oldest known mining deed of the Siegerland. In the Landeskrone lead and silver mine in neighbouring Wilnsdorf, the oldest steam engine operated underground in the Siegerland was working around 1850.
The region's most famous nickname dates from 1789, when the mining official Johann Philipp Becher called the iron mine near Eiserfeld the "queen of the ironstone mines". Later, more than 20 mines in the Kesselborn valley grew together into the Eisenzecher Zug. With a final depth of 1,343 metres, more than eight times as deep as Cologne Cathedral is tall, it was for a time, until 1957, the deepest mine in Europe. It closed for good on 29 February 1960, a leap day, and by the early 1960s iron ore mining in the Siegerland had ended altogether. Geologists also named a stage of the Lower Devonian after the city, the Siegenian. The name is regional, and internationally this interval goes by another name today. Yet our data do not hold a single fossil site within the city.
Who keeps watch today
Mining authorities keep the old mine plans, known shafts are inspected regularly, and where a cavity looks suspicious, drilling rigs and concrete pumps move in. Being honest also means saying that collapses like those in Bochum and Siegen are rare events. The vast majority of the marked points on our map will never turn into a hole, and nobody needs to lose sleep over them. Knowing where they are still helps.
Among the 110 entries there is also an air raid tunnel. The second LWL find report in the city, from 2017, describes a Second World War air raid tunnel in Siegen under the title "Flucht unter Tage", escape underground. Our data add twelve bunkers within the city, all with a hazard marker. For more than two thousand years people took ore out of these hills, and in the darkest years they sought shelter inside them.
Going underground safely
Old adits and shafts are not adventure playgrounds. If you come across a mine entrance in the woods, do not go in, because the hazard markers are there for good reason. If you notice fresh subsidence in the ground, keep your distance and report it. Archaeological sites such as Gerhardsseifen are no place for digging or metal detecting.
There is a safe way in at Siegen-Eiserfeld, where the Reinhold-Forster-Erbstolln has been a show mine since 8 October 1983. The adit was driven from 1805 and served as the main adit of the Eisenzecher Zug, the old queen herself. Guided tours of the first 350 metres run from March to November on the first Sunday of each month. Please check the dates before you go. The orecast map shows what is documented around the show mine in Eiserfeld and near your own street. Does your house stand on old mining ground? Tell us your district in the comments under the video.
Frequently asked questions
What is a Tagesbruch?
Tagesbruch is the German mining term for a collapse that breaks through to the surface from old mine workings. Once a mine is abandoned, the timber props rot, the mine water rises and the roof of a cavity caves in bit by bit. The void moves upwards over decades, often without any sign at the surface. Well-known examples are the Wattenscheid crater in Bochum in 2000 and the Siegen hole of 2004.
How does a natural sinkhole differ from a mining collapse?
A natural sinkhole has no human cause. Where gypsum or other sulphate rock lies underground, water can dissolve cavities over a long time until they cave in. That is what happened in Schmalkalden in Thuringia in 2010, where mining played no part. Such sinkholes are much rarer but hard to predict. Old mining, by contrast, was usually documented on paper and can therefore be mapped.
What was the Siegen hole?
In February 2004 a mining collapse about ten metres across opened on the Rosterberg in Siegen, in Gläserstraße. A birch tree disappeared into it completely, and almost a hundred people had to leave their homes. The cause was the Hohe Grethe mine, which had closed in 1900. The state paid around four million euros for emergency aid and for securing the ground.
Can you visit an old mine in Siegen?
Yes, safely, in the show mine. The Reinhold-Forster-Erbstolln in Siegen-Eiserfeld, the old main adit of the Eisenzecher Zug, has been open to visitors since 1983. Guided tours of the first 350 metres run from March to November on the first Sunday of each month. Please check the dates before you go. Never enter any other old adit or shaft.