Siege of Vraks - The Underground: On Vraks the commanders of the engineer companies were instructed to make preparations for this new phase of the siege. This involved making detailed geological studies of their assigned sectors. Small teams of engineers were sent forward to begin drilling deep bore holes. These would determine the depth of the usable rock strata (and as a useful side effect also locate underground water sources for drinking water for the army), and hence allow them to gauge the suitability of the geology in each regiment’s sectors for offensive tunnelling. It was planned that the tunnelling would ultimately serve one of two purposes. Shallower tunnels would allow for the quick and safe movement of men for surprise attacks (sap tunnels), or deeper tunnels would be dug to undermine enemy defences and position large ‘mines’ - a massive collection of explosives that could be detonated from below, destroying a position ahead of a surface attack.On 812.824.M41 Krieg Engineer units joined the frontline infantry in their trenches and began the systematic geological mapping of the front as the first stage of the underground war that would follow.Vraks itself was not particularly well- suited to large scale military mining operations. Its natural geology meant that shallow tunnels could be dug at great speed as in its upper layers the ancient volcanic pumices were soft and highly porous. Although good for digging surface trenches, for miners the rock was just too soft and unstable. It would be impossible for a tunnel to remain secure and each would soon collapse in on itself. Secondly, the rapid drainage of surface water made it very damp. Tunnels would become running streams in no time, and never have a chance to dry out before another heavy rain storm filled it with water again. The absorption of large amounts of water in a short space of time created a severe problem as it led to expansion of tunnel walls and roofs, placing unpredictable pressures on any supports or roof bracings. It would be like tunnelling through heavy, waterlogged sand - an impossibility.Deeper below Vraks’ porous surface, the rocks began to harden. Here the natural drainage turned into streams and rivers, cutting a labyrinth of underground tunnels and caves. The water carved away at these rocks, draining into ever larger rivers. When these rivers became too large, the cave ceilings above could no longer hold the weight pressing into them and would collapse. This process, repeated over millions of years, had cut Vraks’ surface with many deep wide gorges and canyons. This layer might provide a basis for mining, but again conditions would be very wet and any tunnels risked breaking through into a natural river and flooding, with disastrous consequences for the miners and the mine. It was possible to dig here, but mining would be very slow and dangerous and losses too catastrophic.Deeper still the rocks became harder again, but had the advantage of being strong and dry. Tunnels dug here could be still be hewn by handpicks and, with the addition of some bracing, would have the strength to support themselves. The relative hardness would make the going slow but if necessary machinery could be introduced to speed things up. Of course, this meant that any secrecy would be compromised if the enemy were listening. It was hoped that he was not prepared for an attack from so deep. At this depth oxygen and ventilation would be a problem, and the occasional encounter with the very hard granite-like remains of ancient volcanic activity would bring some tunnels to a premature end, as digging through these would require blasting with such force that a tunnel’s location would be easily spotted from the surface.