Our red marble is legendary, but the black marble is truly unique. The pitch-black marble was not formed on those underwater hills, but in other places, such as lagoons near the mainland. There, organic material flowed into the shallow seawater. Just a few percent of organic carbon was enough to turn the limestone black. Because these places were oxygen-poor, little life thrived there and there are no fossils in the rocks. ‘Belgian black marble is the blackest in the world. Some Arab countries and the United States love it for their palaces and capitols.’

Only one black marble quarry still exists today, in Mazy (in the province of Namur), and that is also the only marble quarry still operating in Belgium today. And whenever something is rare, it's also expensive. Belgian black marble was and is often still laid out in a chequerboard pattern with white Carrara marble from northern Italy.

Geologically speaking, the white marble is more entitled to be called ‘marble’ than its black counterpart from Belgium. Marble, by definition, is a type of metamorphic rock. That means that its crystal structure has changed due to the presence of tremendous pressure and heat: tectonic movements caused the rock to be pushed deep down into the Earth's crust. Belgian marble did not undergo those extreme conditions that existed in Carrara during the formation of the Apennines and the Alps. So our marble is actually an ordinary type of sedimentary limestone. ‘But we can hardly call our wonderful heritage stone 'Belgian limestone', can we?’ laughed Marleen. ‘Marble comes from the Greek marmaros, which means ‘shiny stone’, so in that sense, that name is correct, because the stone itself is polished until it becomes shiny.

Another limestone, one that just about everyone has in their home, is Belgian bluestone, or petit granit. If you look closely at your facing bricks, floor tiles or window sills made of bluestone, you will occasionally spot a fossil shell or a piece of coral and you'll definitely see lots of white flecks. These are remains of sea lilies, which 350 million years ago stood on the sea floor, filtering food particles from the water. They stood together in ‘gardens’, eventually forming thick packages that later petrified to limestone. Break such a piece of bluestone in half, and for a moment it will smell like rotten eggs – the odour of hydrogen sulfide (H₂S), the gas left over after bacteria break down the remains of the sea lilies.

Belgian bluestone is facing competition from a similar greyish limestone from China, but the original stone from Belgium can look back on a rich history in architecture and is one of 55 internationally recognised Heritage Stones, alongside Belgian black marble and Lede stone. Four large bluestone quarries are still in operation near Soignies in the province of Hainaut.

Oxygen-poor oceans were common during the Devonian period and towards the end of that era, around 372 million years ago, the world's oceans were so anoxic that it led to a mass extinction. The Kellwasser crisis, as palaeontologists call it, is one of the great five mass extinctions that complex life on Earth has experienced. The loss of biodiversity was of the same order as the one that took place at the end of the Cretaceous Age, some 66 million years ago, which led to the famous extinction that finally killed the non-flying dinosaurs.

The Kellwasser crisis is easily recognisable in layers of rock by the sudden occurrence of black shale in the form of hardened, fine-grained sea floor with increased levels of organic matter. These point to the lack of oxygen in the oceans. Among the victims were the main reef builders of the Devonian period, the corals and stromatoporoids. But many other forms of marine life were also severely hit, including brachiopods, trilobites, ammonites, conodonts and armoured fishes. ‘Oxygen-deficient zones in seas and oceans still exist to this day,’ said the palaeontologist Stijn Goolaerts of the Institute of Natural Sciences, who has launched a research project into the Kellwasser crisis in Belgium. ‘Usually, these zones are local, short-lived and exist in only one section of the water column. Only when those oxygen-deficient zones increase in area and number, persist for a long time and begin to encompass larger portions of the water column, do they make life impossible.’

During the Kellwasser crisis, the lack of oxygen lasted several hundred thousand years, and manifested itself in two pulses, the second being the most pervasive. At the quarry in Lompret, where Natalie and Kevin found their large armoured fish, you can extract plenty of samples from that interval.

So how did those oceans become deoxygenated? Perhaps it was due to algal blooms growing in overdrive. Due to the advent of land plants, many more nutrients flowed seawards. Rivers carried fertile soil that contaminated the seas and oceans, causing massive algal blooms. When those algae died and sank to the bottom, so much oxygen was consumed by bacteria during the decomposition of those algae that other organisms that needed oxygen suffocated.

Scientists have other possible explanations for the Kellwasser extinction, including massive volcanic activity in Siberia, which would have blown large amounts of greenhouse gases into the air, altered the climate, and also, among other things, increased the quantity of algal blooms in the oceans. This may also have been accompanied by a change in our orbit around the sun. Periodically, the Earth has a circular orbit rather than the usual elliptical one (the Milankovitch cycles) and orbits that are more circular lead to a more stable climate and slower ocean circulation. This means that oxygen-poor oceans will then remain oxygen-poor for longer. ‘No global extinction is due to one individual cause’, said the geologist Xavier Devleeschouwer of the Institute of Natural Sciences. ‘The combination of causes is what makes the cocktail toxic and pushes entire ecosystems over the edge.’

Thirteen million years after the Kellwasser ecological disaster, life received a second uppercut: a global cooling known as the Hangenberg Event. ‘The earth tumbled from a warm period into an icehouse. The world changed dramatically, with ice caps at the poles. We can see geological traces of that in South America – the moraines. These consist of the debris that a glacier carries with it as it moves and leaves behind when it melts or retreats.’

How the earth ended up in an ice age is again up for debate. Was it caused by increasing plant growth on land? Plants remove CO₂ from the air, thereby causing a reverse greenhouse effect. But that seems too simple an explanation. One thing is certain – the ice caps caused sea levels to drop, leading to another ecological shift in the oceans. The Hangenberg cooling was now shaking things up on land as well. Palaeontologists therefore think that there must be at least one more cause. They have evidence of an enduring hole in the ozone layer, resulting in increased UV radiation and damage to the DNA of land-based organisms.

In the aftermath of two major waves of extinction, one due (among other things) to oxygen depletion in the oceans and one due (among other things) to global cooling, the Devonian period came to an end and the Carboniferous period began. Life was going through a bottleneck, but came through as it always does. This new era would give the planet its first truly impressive forests, which would later petrify into coal. For us human beings, this was a blessing, as the resulting coal was the driving force behind the Industrial Revolution, but it was also a curse, as the burning of fossil fuels caused the global warming we are witnessing today. The creatures that were swarming, crawling and flying around in those primaeval forests of the Carboniferous period defy imagination. It was certainly no place for anyone with a phobia of spiders or insects!