All it took was one wrong step.

Thomas had tried to follow instructions, to keep stepping in the same spots as his friends, but his legs just weren't long enough, and his foot had slipped in the slick mud. He'd slid across the wooden slats like a hockey puck on ice, but then it crumbled under his weight and he fell.

Thomas had heard Percy's wail of his name as he plummeted into the dark, but then it was cut short as he splashed into water that must've collected under the wood over time. His body stiffened at the cold and sudden impact, preventing him from moving until the surface was well above him... Somewhere. He was sinking, slowly falling, but it was pitch black around him, he didn't know which way was up or even how to get there. He was an engine, and engines weren't taught how to swim.

His arms and legs remained limp as he sank, the pounding in his chest fast and hard. H-Help! Help! Help me!
He didn't know who or what he might be calling out for, his friends couldn't hear him down here. His head was spinning, his thoughts swirling...

Then a light appeared above him. Blue in color, it seemed to ignite the depths as it approached, allowing Thomas to see around him. It then took on a shape, the shape of his oldest friend diving down after him. E-Edward...!?

Hang on! Edward's voice seemed to echo despite his lips not even moving, his eyes glowing brighter as he reached out and grabbed Thomas around the torso and pulled him closer to him. Hold on...!

He twisted to help Thomas lay on his back and wrap the little engine's arms around his neck, and then somehow knew how to swim back upwards. Kicking his legs, the larger blue engine moved higher and higher before both of them gasped for air as they reached the top.

Thomas heard someone holler from above, and then from the hole in the wood came a familiar hook and line curtesy of Cranky. Edward swam over to the line and grabbed onto it, and then Thomas heard the crane grunting softly, and the line Edward grasped began to retract, lifting them from the water and back into the dim light from Boulder and Blades' lights.

"Thomas!"

Thomas looked as Cranky's crane arm retracted backwards, pulling them away from the wood and onto solid ground. Mud and soil caked the crane's lower legs, as if providing more leverage as he twisted and then placed Edward down carefully on the ground. Thomas was shaking as Edward sat down on his hands and knees, but he smiled as Percy rushed over and grabbed onto him.

"Thomas, you're okay!"

"Y-Yeah...!" Thomas hugged his friend back, "I-I'm okay...! Thanks, Edward... Thanks, Cranky..."

Cranky gave the little engine a slight smile, and then helped Edward- whose eyes had gone back to normal and now appeared a little dazed- up to his feet. "Try not to do that again, Thomas."

"A-Agreed...!"

"You two alright?"

Edward slowly nodded, and Thomas tried to stand up, but his legs wobbled underneath him and he fell back down into the mud. "N-Not quite ready to walk, I guess..."

Edward looked to him and then offered his back once more, "Don't worry, Thomas... I can carry you-"

"Edward, what were you thinking!?"
Gordon marched over to the other blue engine as he adjusted Thomas' position on his back, "You could have easily gotten both of you trapped down there!"

Edward tilted his head to one side, "I didn't really, I just... Acted on instinct."

"Good instincts, then." Blades praised.

Blades and Boulder pounded their fists together, and then Boulder overlooked the group. "Okay, let's keep going... I might just have Graham groundbridge us out of here when we're done instead of trying to navigate that hole again..."
Boulder clapped his hands again, feeling the tunnels ahead. "...It splits just ahead, but after that, no splitting up."

"Agreed!" Diesel informed, "No more falling into giant holes either!"

Percy stuck particularly close to Edward as they continued on their way, helping him brush what mud was still stuck to Thomas' framing after his sudden bath. "You sure you're okay?"

"Startled... But fine." Thomas mumbled, "That water was cold...! And I couldn't see anything down there. It was all pitch black... I was falling, but I had no idea which way was up or down...!"

"If anyone was going to fall in, I'm glad it was you two." Cranky admitted, "Imagine me trying to haul Gordon out on my own."

"Heh. Nah, we'd just have to come back for him." Diesel teased.

"Don't even joke." Edward hissed, "We shouldn't leave anyone behind."

"Did you guys have to deal with sinkholes much, Boulder?" Henry asked.

"Not a lot," Boulder admitted, "Though there was that time when we were underground trying to get to an energon deposit and the ground broke out from under us. We all got separated and had to find each other again."

"Not a fun time, that one." Blades admitted, "But my rotors bent and couldn't hardly fly. There was also this massive fish in an underground lake down there that tried to eat me."

"Is it still down there?"

"Eh, who knows? There's been enough quakes since then that it might have found a way out to sea now."

"And then there was that time Chase and Chief Burns fell through the wooden floor in the lead mine and wound up wandering through there all night." Boulder reminded. "Thank Primus that time the ground under the wood was dry, or Cody would've never seen Chase's footsteps in the dirt to tell us where they went."

"What's the biggest rescue mission you've had to do?"

"Hmm... Tough to say, there's been a lot. I mean, there was that time the entire island was sinking into the ocean... Twice, actually."

Diesel puffed, "Yeah, well around here, we just do our jobs and then- Gyah!"

"Diesel!" The other engines all turned as they heard him cry out and then it was drowned out by the sound of crumbling and tumbling rock. Percy led the charge, "Diesel, are you- Whoa!"

"Watch out, Percy!" Thomas yelped as Edward grabbed the little green engine's arm and yanked him back from yet another sinkhole in the floor of the shaft. In the darkness, just like the one under the wooden slats, it seemed to go down forever. "Diesel!? Diesel, are you okay!?"

Boulder lifted his hand and then the ground began to shift- slowly, a set of steps appeared on the edge of the hole, and then he started down them, and the engines cautiously followed. "Diesel?"

"Ugh... Ow."

As their eyes adjusted to the darkness, they could make out Diesel pulling himself up from the ground. "What was that you just said about not falling into any more holes?" James teased.

"Oh shut up, James."

"Are you hurt?" Emily asked.

"What? Me? Hurt?" Diesel defended, "N-No! I was just- AAAAHH!"
The diesel engine stumbled back from where he'd been in pure panic, and Boulder's lights ignited as he caught him and put the younger mosaic behind him. "Cinders and ashes!"

"What?" Emily asked worriedly, "What is it?"

Boulder's lights shone in front of them, casting beams of brightness that illuminated the stone wall ahead. Resting in front of it was a blackened pile of something, twisted and crumbling.

"Cinders and ashes...!" Edward whispered in horror, "Is that... or was that-"

Boulder took a slow step towards the shape, his optics settling on it for a moment before his shoulders slumped, and he nodded. "...It was a human."

The other mosaics backed away, looking away or covering their mouths in shock. "What happened to them...?" Percy whimpered.

Boulder bent down and then waved a hand over the body, a wave of soil and rock washing over it to properly bury it. "...I don't know, but it looks like they got caught in an intense explosion of some sort, would have killed them instantly."

"What could have caused something like that...?" Cranky murmured, "...Hey, look at that."

Boulder looked back, following Cranky's gaze to the wall. The stone around them had fractured, and inside of the cracks... Boulder could see a faint, blue glow.

"Is that the new mineral they mentioned?" Henry asked.

"What kind of coal glows?" Diesel asked.

"That's not coal..." Boulder whispered, walking over and disabling his lights to get a better look at the glowing mineral inside the cracks. "...That's energon."

"I thought energon was a liquid?" Emily asked.

"Well, it is, but only after it's been refined. Before that, it's in this mineral form embedded in rock."
Blades hovered over to the edge of the cave, feeling the stone with a hand. "How far out do you think this vein goes?"

"Let me see..."
Boulder walked over to the wall, putting both hands on it. His eyes closed, and then after a moment, he stepped back. "...It's... All over the island. This mine is the center, and then it just branches out... like a spider web... The note."

"What note?" Thomas asked as Boulder reached into his cab.

"When I investigated Sir Topham Hatt's office, I found a journal he left, as well as a paper..." He unfolded the paper. "Strange New Mineral found in Southern Mine Shaft. Investigate further. Hatt."

"So the energon is what they found?" Henry asked.

"Looks like it, but energon is highly combustible. If you hit it wrong and it ignites-"

"Boom." Gordon muttered with horror, looking to the mound where the human's remains were now buried.

"Exactly..." Blades flew up and examined the walls and ceiling, "Looks like it was a small, but powerful explosion... The humans must've been trying to get a chunk out for study."

"And after the explosion, they would have shut down the mining until they knew how far out the energon went." Boulder rubbed his chin, "And when the government found out about it, they condemned the island as a safety hazard to the civilians and ordered an evacuation...!"

"And that would be why the humans left!" Edward exclaimed, "And if they found out that the energon was not of Earth, they would have kept it a secret from the public, to avoid people coming out here to snoop and putting themselves at risk."

"Which is why we have no record of why the humans suddenly left the island. It was all under lock and key..." Cranky hummed, "...Now it all makes sense..."

"Eheh... We did it...!" Thomas announced nervously, "We solved the mystery...Yay...!"

"Perhaps, but it still doesn't fully fill in the gaps." Boulder's brows furrowed, "We need to talk to Chief Burns. He can contact Fowler, see if we can get any more information from the UK officials about what happened."

"But how could an entire island on Earth become infected by energon veins this deep underground?" Emily asked.

"A long time ago, cybertronians hid stashes of liquid energon they'd refined on other worlds. Over time, the energon must've spilled and returned to its crystalline form."

"Hey," Blades announced with a grin, "At least it isn't Dark Energon...!"

Boulder smirked, pushing his brother's wiggling fingers away from him. "Come on, Blades. What would Dark Energon be doing on Earth?... Let's get back topside."

"Gladly!" Diesel announced, running towards the steps Boulder had made. "Last engine out's gotta pull The Flying Kipper all day!"

"Oh you know I'm not pulling the Kipper!" James announced.

"N-No running on the stairs!" Blades warned, "You could hurt yourselves!"

"Look at you, Mother Hen..." Boulder teased.

Blades huffed, his antenna drooping to the side a little. "Hmph... Not a mother hen..."

Boulder smirked, and then tapped his comm. "Graham? I think we're done here for now. Do you think you could send us a groundbridge from here back to Griffin Rock? We'd like to stop falling into holes down here."

And from the top of the hole came a collective of voices. "YES PLEASE!"