D Genesis: Dungeon ga Dekite 3-nen

Three years ago, dungeons suddenly appeared on Earth after an experiment in Area 51 went awry. Now everyone—from average citizens to soldiers—explores these monster-filled labyrinths in search of wealth, power, and magic. Keigo Yoshimura is an office worker with no sense of adventure, who dreams of quitting his job to live the easy life. While out on a business call, he stumbles on the birth of a new dungeon and accidentally gains a magical skill that turns subterranean exploration into an RPG. Things spiral out of control and Keigo winds up as the world's top explorer. With help from Azusa Miyoshi, his mathematically gifted colleague and new business partner in dungeon diving, he might be able to turn the status screens he sees into piles of cash. Unfortunately, Keigo ends up under the scrutiny of the military, government agencies, and even more sinister forces. What happened to his dream of taking it easy?!

(Source: J-Novel Club)