Small dots moved across the sky just beneath the surface of the Dome Shield. Their long spiral paths travelled south in lazy overlapping patterns. The reconnaissance drones had been searching for more than twenty hours, building up a map of the density of the subterranean rocks. Their paths had already taken them over large areas of the Orange Zone. It would take them over a week to complete their scan but interesting data was already starting to arrive.
The drones used gradiometry to detect minor changes in the strength of Megarothia's gravitational field. The strength of the field was thought by many to be uniform, but it actually varied according to the distance from the planet's core. The force of gravity was weaker in the upper atmosphere than it was on the planet's surface, and many kilometres underground the strength of the gravitational field would be stronger still.
The original detectors had been used by mining companies, looking for soft minerals underground, but Parker had managed to modify the equipment, making it far more sensitive. The detectors could now identify tiny differences in the gravitational field across areas smaller than a metre across. The drones had been programmed to overlap each other's flight paths, combining their data into a single model of the gravitational field strength at each location on a three-dimensional map. Tiny variations in the magnetic field strength correlated with the variations in the density of the subterranean rock. The model was already beginning to take shape, and the drones' overlapping flight paths were helping to cancel out any erroneous readings.
The map showed a number of underground caverns to the south of Cinnamon City. The ones Parker had seen so far had been irregular in shape, dropping a hundred metres down narrow vertical tunnels before twisting at odd angles and opening out into deep underground caves. These were natural caves, formed by erosion and running water. One of the caves was much larger than the others, and much deeper too. It was known to be one of the original fless caves, and it had been empty for hundreds of years. The quality of the data was excellent. Parker knew he would be able to find what Rachel was looking for. The drones just needed a little more time to complete their scans.