Hajime Naehara 
A Cave King’s Road to Paradise: Climbing to the Top with My Almighty Mining Skills! (Manga) Volume 4 [EPUB ebook] 

With the leviathan slain, peace has returned to Sheol. Everyone gets to catch their breath and recover while welcoming the new additions to the community. That is, until exciting discoveries underground appear out of nowhere!
The first on the list is a hot spring—and who can say no to a nice bath? Certainly not the residents of Sheol, so it’s time for yet another grand construction project! The second is a long-anticipated discovery: more ruins, this time equipped with ancient technology. Finally, Heale gets a glimpse of the island’s mysterious past…which might have heavier implications than he ever expected. But there’s no need to fear, because his trusty comrades have some surprises of their own…