passengers) and the white foam-board scale model that took up the length of three desks. Two junior engineers â Ruairà and Deirdre, both fresh out of college â were on hand to answer any of his questions about the site.
Ruairà was quite short and unshaven, with a ruddy complexion. It was clear from his overhanging pot-belly that he enjoyed a little more fast food than was healthy. Deirdre, meanwhile, was much taller, with a slender build and her hair tied in a bun. She wore thick, round glasses and it was clear to Joe from the moment he met them that Deirdre liked Ruairà quite a bit but Ruairà was too wrapped up in his work to notice.
Joe finished scrawling a quick note for himself then tapped a smooth mound on the model. The mound had little foam paving stones to indicate the pavement that would eventually be placed there, as long as things went according to plan.
âSo this is where the latest cave-in was?â he asked as he tapped.
Ruairà nodded. âThatâs where the latest â and worst â cave-in was. Though thankfully no one was hurt.â
âHow can that be?â
âThat no one was hurt?â
âNo. The cave-in.â He pulled out the geothermal mapping for the mound area. The red and yellow swirls on the image showed layers of mud, bedrock and limestone below ground level. The mapping went three hundred feet underground, deeper than they were drilling for the tunnel. If something that would cause a cave-in had been there, it should have shown up on this image as a black space between the waves. Yet the image showed that the ground should have been solid rock all the way down. âThereâs nothing here that indicates a cave-in was even possible, let alone probable.â
The engineers looked at each other in silence. Joe caught their look.
âWhat is it?â
âWell,â said Deirdre, âthereâs more â¦â
In the open as they were, the wind howled against them, chilling them to their bones. Joe and his junior engineers were standing beside the mound in bright yellow hard hats. Tape cordoned off the mound, which was as even and smooth as it appeared in the model, as if no cave-in had happened underneath.
âHow did you manage to fill it in so neatly and so quickly?â Joe asked, astonished.
âWe didnât,â said Deirdre.
âThen how â¦?â
âThis is how it was. This is how it always was. Even during the cave-in.â
âWhat do you mean âduring the cave-inâ?â asked Joe.
Ruairà and Deirdre looked at each other glumly.
Eventually Deirdre continued. âIâm guessing no one explained the true nature of the cave-in?â
Joe shook his head, intrigued.
âBoth of us were here that day â and about ten other engineers and excavators. We were starting a small dig on the mound, just some tests. Suddenly the ground started shaking. We could feel it underneath us, rocks falling away. We could even hear them crashing down below the surface.â
âWe have some experienced excavators here,â Ruairà added. âThey all said the same thing: that it felt like a cave-in.â
âSo we ran to safety,â said Deirdre. âAfter a few minutes the shaking and crashing stopped. We waited a few more hours before venturing closer again. And this is what we found: the mound just as you see it now.â
âI donât understand. If there was a cave-in then all the rock from the surface should have fallen through. This mound shouldnât be level.â
âExactly,â agreed RuairÃ. âBut whatâs even more confusing is that when we did another geothermal it showed that itâs completely solid the whole way down â as if there had been no cave-in.â
âThen itâs faulty mapping.â
âItâs not faulty. Weâve tested it elsewhere. And weâve used different machines on the mound. All of