Another time skip:

"G, can you hurry? It's so hot here..." Your skin was glistening from the heat that was caused by the lava pools beneath you. You and Gaster were in the part of the cave that held those giant pools of heat and death.

Being a skeleton, Gaster had no problem withstanding the heat but you felt like you were melting and burning alive at the same time. 

Occasionally, you had to step out of this place to the cold and awfully snowy parts of the cave to cool yourself down or else you would surely be suffering from a heat stroke or the like.

Gaster's magic hand was tugging on the bottom of your shirt to get your attention and you turned to him to see Gaster kneeling right near the edge of one of the many cliffs in this place.

Your heart skipped a beat and you immediately shouted: "Are you crazy?! Get away from there!"

But he paid you no mind, fumbling around on his devices instead. As he untangled the wires and did some things here and there, his magic hands signaled you to come closer.

Carefully, you stepped up to him, trying to look over the edge into certain doom. 

"I swear to you, if you push me or try anything funny. I do NOT feel up for funny.", you growled but Gaster got onto his feet instead and gently wrapped an arm around you to carefully guide you and pointed down the cliff. You opened your mouth in both surprise and awe. 

The lava wasn't just normal lava how you knew or imagined it. It was glistening and shimmering in hundreds of vibrant colors while still emitting the warm red-yellowish glow of typical lava. You both stepped back again to get a safe distance between the cliff and yourselves before you began to speak: "That...is beautiful." 

'And our future.' 

He picked up all his devices to pack them up, signing to you with his magic hands: 'This lava has magic properties which I will continue to explore. I was able to replicate human electricity by making devices compatible with a monster's magic. So right now, when you touch the device, you may power it. But what if we got a reliable and endless source instead? What if we had another source to get this kind of magic from?' 

"You mean...the magic lava." Gaster turned to you with all his things in his arms and beamed at you: 'Exactly! If I can find a way to make use of all this energy, I could power up the whole Underground the way humans do with their electronic devices and plugs. We can bring this energy to everyone. I just have to create a plan.' 

"That's amazing, Gaster! I will support you however I can!" 

Gaster was already deep in thought and seemed to have missed what you told him, but you just smiled at him and let him ponder in silence as you walked with him to return to Home.

.

.

.

Just a couple of days had passed and the whole Underground heard of Gaster's discovery. Of course, he didn't waste much time to draft blueprints and create new ideas to utilize this magic lava somehow. The development and following installations proved to be challenging due to the lava's intense heat. Finding the right materials and safe ways for the installations was quite difficult, considering Gaster did not want to put anyone at risk for his own inventions and maybe somewhat crazy ideas.

The ideas he had were brilliant by itself, but not everyone was on board with Gaster's enthusiasm, still fearing the reappearance of possible human threats.

Monsters were worried that if Gaster would tamper around too much, he could draw attention to the Underground and the safety of being forgotten by humans might be taken.

Gaster's ideas were shut down pretty quickly but he still did not stop on researching the possibilities – maybe for someday in the future. You thought he just finished with his sudden flow of ideas and innovations when he suddenly held up a new blueprint into your face. It looked like a giant construction, almost like a building. 

"What is that?", you asked and took the paper from him to inspect it more thoroughly. Gaster gave you a second to look at everything, then started signing when you shifted your attention to him: 'This is a blueprint for my lab.' 

"Your lab? But you already have one. Isn't that sufficient enough?" 

'It may be but imagine a place with unlimited energy. I could build bigger machines that require a much bigger charge than those small devices. Think of all the possibilities – the discoveries. Maybe they could help us break the barrier.' 

You frowned as he mentioned the barrier. "Why do you want to break out so badly? There is nothing for us out there and in the worst case, they will just chase us back down here again."

Gaster took the blueprint and rolled it up, clenching his hands around it. The paper rustled, but he remained silent, staring off into the distance.

With a sudden movement, he adjusted his glasses and smiled at you apologetically: 'Yeah, you are right. It was a foolish idea of mine. I apologize.' 

You did not like his sudden change of mood but it felt like it was wrong to ask so you just let it slide, nodding at him in silence.