The first Exploitations
The Celtic Parisii were probably the first to exploit the many minerals present in the Parisian basin. Clay, known for its vessel-making qualities, could be found on the surface on the Left Bank in the banks of the Bièvre River. Sand with which it was often mixed was washed inland along the Seine shoreline where it was prone to flooding, that is to say in the lands that is today's Grenelle and Auteuil, and was also present in its natural state of stratification atop the highest of Paris' hills. Paris, having spent much of its geological life between lake and sea, also had a vast deposit of high-quality sandstone, but stone in those times was used roughly as walls and foundations, and was most likely also brought from where it could be found on the surface.
The Romans would be the first to launch the industry of mining Paris from 52 BC. Because of their technical knowledge they found use for most all of Paris' minerals, and even knew where to find them if not visible from the surface. Clay they used of course for pottery, but also for other architectural uses such as roofing tiles and pipes. Sand they used even to make glass. They also had use for the gypsum found in the higher reaches of the Right Bank hills of Montmartre and Belleville for its qualities, once baked and pulverized, as plaster. Though it is still a subject of dispute today, their name for Paris, "Lutetia", may have its origins in "Lutum" meaning Latin mud or clay.
The first technique used to mine minerals was to simply gather them from where it could be found. Once this means was exhausted, the next step would be to try to dig or levy the deposit from its geological formation. This could be considered as the origins of mining.
Pit mining was the most efficient means of extracting a mineral as it was done in open air. If a targeted mineral was found close to the surface, all layers above it would be removed over the intended area of exploitation. A level of mineral would be removed to a regular depth, and once this was exhausted the extraction of the next below would begin but at a circumference smaller than the last. This at once ensured the stability of the excavation and assured a simple means of raising the lower level product to the surface. Early evidence of this type of mining have been found in several places in the Parisian basin, and it is even thought that the Left Bank "Arènes de Lutèce" was, as the foundations of its construction, an exhausted pit mine.
Romans had understood the workings of stratification well before their arrival to Paris. It is more than likely out of a question of convenience that, instead of extracting clay horizontally from the side of the hill where it could be found to the south of their town, they dug down vertically to find it at a location closer to the city centre. While sounding the ground for the construction of the Left Bank Pantheon, explorers found archaeological evidence of Gallo-Roman mining in cavities 30 metres below the surface. The quality of clay there was not the best but obviously had its industrial uses; called "glaises vertes" in the French geological jargon, it was a result of freshwater sedimentation mixed with organic matter that sometimes could contain noxious or even flammable gases. This would limit the distance from which one could mine horizontally from the shaft's bottom at where the bank was found, but the elasticity of the material also had its conveniences, it had a tendency to shift and fill any cavities created in it. In this way, a miner, having exploited as much as he could of the bank around his shaft, need only close it and wait for the clay to return again. This was Paris' first-known vertically accessed mineral exploitation.
Between all of the above there were of course mines where excavators would dig horizontally into the mineral mass from the side of a hill in a technique called "bouches de cavage". After a certain horizontal distance we can consider this type of exploitation to be the same as the one just above, using techniques more advance that we examine in the section following.
Lutèce saw its peak growth in around 200AD, covering a terrain forming a rough diamond whose opposite points were at (roughly) today's place du Châtelet and the Val-de-Grace hospital. Even common Roman buildings were built in stone (save for their roof), and it is thought that the town had exhausted most all of the Parisian basin's pit mining sandstone by half that time.
When a wanted mineral lay too far below the surface for it to be extracted over a wide area from above, a miner had to turn to other means. Digging into the side of a hill along the mineral strata was the simplest means, but the cavity created by its removal would cause an increasing question of stability as the extraction progressed. The ceiling above had to be upheld somehow, and over the centuries miners developed a few techniques that more or less assured the mine's safety.
The earliest means of excavation and extraction of a given mineral was to dig into it in a straight hallway, or dig down to it and do the same, then to bisect this hallway with several lateral others. These in turn would be bisected with halls of their own, and the result would be a gridwork of halls divided by square columns of unexploited mineral called "piliers tournés". As you can imagine, although they were the ceiling’s only support, were considered by the mine's owner as a "loss". For this these hallways would vary in width depending on a) the hardness of the mineral exploited b) the mine owner’s daring and c) the mine owner’s honesty. Some extracted in swaths wider in normally possible, but tried to compensate by interposing columns of un-mortared rubble blocks (called "piliers à bras" in the French mining jargon), but this could be little more than a temporary measure.
A more efficient "hagues et bourrages" mining technique appeared from the 16th century. It involved removing absolutely all the desired mineral over a certain area and replacing it with rubble and landfill; miners would dig forward into the bank a certain distance, then outwards in all directions forming a room of a certain breadth. Once the exploitation completed and the landfill would be packed into the cavity to both sides of a central hallway, the form of which would be completed with dry masonry, and once this complete the digging would go forward again to another area. This again was fine for the mine's foreseeable future, but, even as we can see by certain hallways today in Paris' underground, this landfill tends to settle with time, leaving the ceiling unsupported.
The above describes mining only over one level. Imagine the same over several: Paris' sandstone deposit reaches thicknesses of over fifteen metres in certain places under its left bank around its rue Vaugirard, and all of it has been exploited. Oddly and perhaps luckily enough, this "assise" of building-quality stone is divided into three by two lesser-quality levels of hard rock, and these make ceiling and floor for the centre level. Communication between the three was made possible by ramps, secondary wells or masonry stairs.
Malleable clay was the simplest mineral to extract and was extracted using all techniques described above. Sandstone mines within Paris used only the "piliers tournés" and "hagues et bourrages" methods as the mineral existed close to the surface only in inhabited regions.
The first known technique of extracting sandstone was to simply widen its existing fissures and cracks to try to obtain a rough block. Later innovation took advantage of the wanted stone’s own weightWorkers would dig into the bank (preferably) into the lesser quality stone below what was to be the block extracted, replacing it with wooden supports as they dug. Once enough rubble had been removed over a large enough area, the blocks (sometimes poles) would be removed and the stone chiseled where the miners wanted it to break under the weight of its own gravity. It then would be dragged forward onto a skid or cart to be dragged to the mine entry. If the mine was shallow and opened to the side of a hill this was a simple affair, but if it was deep or accessible only through a well the stone had to be raised to the surface by other means. For this was invented the "roue de Carrière", a mouse-wheel like contraption that was actioned by a miner "walking" its rungs with the load-lifting cord wound in around its axis. This was a delicate job, as any slip would mean a danger to both the wheel-walker and the miners below.
Gypsum was quite another affair. Present in Paris' hills of Montmartre and Belleville, it existed in thicknesses reaching fifteen metres in places. As it lay quite near the surface to the southern reaches of the latter hill, it was exploitable in open-air pit mines, but where it ran deeper under the hill's earthy cap, it had to be exploited in the "piliers tournés" method. As the gypsum bank was much thicker it was extracted by adding another step to the process described above: it would be mined from the top downwards. Once in the upper reaches of the desired mineral, miners would dig hallways that, although narrow in the first level of excavation, would gradually widen as the digging move downward through the mass. The result was a high cavity with columns that widened as they rose towards the ceiling. In Roman times (as Paris' gypsum was exploited even then) blocks of mineral would be hacked out of the mass in rough blocks, but from its invention, dynamite became a quicker solution. The process remained the same, save for heavy wooden girders forced into the walls near the ceiling of the first level of exploitation: these would reinforce the tip of the arch and uphold blocks shaken loose from the ceiling under the force of the explosion.
A Plan for Disaster
Unmentioned in all the above is the fact that early miners weren't much in the habit of drawing plans. Once into a bank, a mine owner often just followed the mineral as far as he needed or until the quality of stone became undesirable. Although most often begun from an isolated spot, once the work had begun little or no thought was paid to the world above as it progressed, which often meant that mines undercut properties, buildings and roadways.
The above became a real problem once a mine was abandoned. Those using the "piliers tournés" technique were relatively solid (column density depending) and the extent to which they were mined was quite obvious. This was not the same for mines using the "hagues and bourrages" method as its extent of exploitation was hidden behind tons of landfill.
A long-abandoned mine would eventually begin to succumb to the forces of nature. A humid underground climate engenders expansion, corrosion and oxidisation, and these could eventually end up causing shifts in the ceiling mass that would cause fissures in the weaker strata retaining it from above. If this process went far enough a thickness of rock would fall, creating a "medallion" cavity; should this process repeat itself the next layer to fall would be of a smaller circumference. The result would form a dome called a "fontis" - and should the diminishing erosion not reach a form of stability before it reached the surface, the ceiling would fall in and the mine would be open to the world above.
Gypsum mines were even more fragile faced with time. It is a mineral extremely sensitive to humidity, and after a period of time exposed to the air as roof-bearing columns it would begin to degrade. What may have even added to the mine's future instability even during its time of exploitation was the practice of housing the plaster ovens (used to bake the gypsum into this commercial form) within the mine itself, causing enormous heat and humidity variations. If a weakened column should fall, the fontis phenomenon described above would begin. If the mine were a hastily or badly planned one, a fontis would begin on its own.
The above processes are slow, and often it would take centuries for them to affect the world above. Luckily for Paris, she wouldn't begin to suffer the effects of a forgotten and haphazard mining until well into the 17th century.