In the dead of night, Sakura, Minato and the Hokage Guard Platoon ventured forth from Mount Shumisen into the wasteland, following a certain tip from a man who claimed to be Orochimaru's best friend. His messenger hawk had arrived earlier in the evening, bearing a plea for help from Tonika and Hachō Villages and confirming the intel Sakura had extracted from the three Iwa-nin she had captured.
The enemy was indeed in the Hole!
"Area Scanning Jutsu!"
At some point, having seemingly found the spot she was looking for, Sakura halted in her tracks and slammed her hands against the ground, chakra rippling from her palms in pulses of varying frequency. The waves sank into the arid soil, probing deeper as they went, rebounding off subterranean structures and refracting through phase changes within the earth itself.
"Not bad, daughter-in-law," Minato said with a low whistle. "That's a neat trick. Did Tsunade teach you that?"
Sakura nodded.
Long ago, after fighting Kakuzu and losing her breasts, she had been hospitalised and bed-bound for a week. Tsunade had taken that opportunity to teach her the technique, since the most difficult part of the jutsu was making sense of the garbled stream of gibberish it produced.
Although this knockoff sensory jutsu did not appear, at first glance, to be especially impressive, and the effort Sakura had put into learning it had previously been greatly glossed over, it was in fact one of the most taxing techniques in her arsenal. It required exceptionally fine chakra control to produce both seismic and electromagnetic waves, as well as memorising detailed knowledge of every type of soil one might find across the shinobi continent.
As Sakura wasn't a sensor-type by nature, unable to sense the presence of airborne chakra in weak concentrations without first entering Sage Mode, this jutsu suited her perfectly, since she had both control and intellect in abundance!
Sakura's brow furrowed in concentration as she read the pulses of chakra returning from deep underground. There was no way to verify her readings without physically tunnelling down for soil samples, but by measuring the shear-wave velocities of the subterranean layers, she could infer the materials they were composed of. From there, she could draw on her book knowledge to estimate the probable dielectric permittivity of inferred soil types, to build in her mind a clearer, higher-resolution picture of what lay beneath her using electromagnetic waves.
"Very dry soil," Sakura muttered under her breath. "Strange for this geographical location, but it does make the mental arithmetic easier…"
It had been bothering her for a while, now.
"It's not as if this place never gets rain," Sakura said to herself quizzically. "So why is it all lifeless wastes, except for the Hole?"
Mount Shumisen was an old, extinct volcano, which meant the surrounding land ought to have been exceedingly fertile, much like the rest of the Land of Fire, and yet, despite receiving water run-off from the Land of Waterfalls, the north-western reaches of the country were a dead wasteland. The nature energy here was so thin as to be almost non-existent, which, to Sakura, was frankly perplexing; even the Land of Wind's deserts held more of it than this place.
"Found it!" Minato said as he rose to his feet and pointed to the south. "The tunnel that Disonasu-dono mentioned is thirty metres deep, five paces that way."
Seeing that Sakura was taking her time, Minato had taken it upon himself to try his hand at the Area Scanning Jutsu as well. Of course, she could also have used the Byakugan to obtain an answer straight away, but she didn't like using it in front of people since she already had the Sharingan.
"I've got thirty-one metres," Sakura said reflexively. "But close enough."
Which brings us back to the topic at hand:
Why were Sakura, Minato and the Hokage Guard Platoon wandering around the wasteland in the middle of the night and scanning for subterranean structures? That was because they were following up on the tip sent to them by Orochimaru's self-proclaimed best friend, the leader of Hachō Village, a certain fellow named Disonasu.
"I would hesitate to call Disonasu a friend, but I can certainly recognise his expertise in summoning techniques," Orochimaru had replied over the radio from the north-eastern frontlines when questioned about him. "It's thanks to his help that I finally managed to perfect the Edo Tensei…"
As the world's foremost evil scientist, Orochimaru had many such acquaintances, including Dr Shinnō and Amachi. The former was a reformed evil doctor whose motto had been First Do Harm, while the latter was a marine biologist who had transformed both himself and Isaribi-chan into fish-human hybrids, so Sakura was not surprised that the Great White Snake somehow had a hand in this as well.
"Why is Disonasu spending his retirement leading a village in the middle of nowhere?" Orochimaru had said when probed about Disonasu's puzzling career change. "I have no earthly idea, you will have to ask him yourself. Perhaps he was simply tired of this life of ours. He was never quite himself again after Pain crippled him…"
Since Iwagakure's attack had come without warning, Disonasu's letter was very brief. Hachō Village's self-defence force had bought him just enough time to describe how they might sneak into the Hole through the network of tunnels that ran through the wasteland. Apparently, these tunnels were shielded, meaning the enemy's sensors would not be able to detect anyone arriving through them.
"Earth Style: Hiding Like a Mole Jutsu!"
As one, Sakura and the others activated the jutsu, causing the dry, lifeless soil beneath their feet to turn easily into sand, allowing them to sink into the ground and swim towards the strange structure her Area Scanning Jutsu had pinged. A few seconds later, they emerged before a large black pipeline, roughly the diameter of a tall man.
Sakura created a small pocket of air around them and the black tube's side.
"Well, well, it really was here…" Genma said, taking the senbon he perpetually held in his mouth and prodding the pipe with it. "And what's it made of? I can't even leave a scratch on this thing…"
"Crystal Style: Diamond senbon!"
Sakura produced a crystal senbon and tried to score the pipe's surface with it, but to no avail. Whatever mystery material the black conduit was made of, it was somehow even harder than diamond. Only when she added wind nature transformation to her chakra and coated the needle with it did she finally manage to carve a groove into the inky-black tube, but, to her surprise, the thin line quickly grew fainter before disappearing entirely.