Relirium -Reririum- Ruins and Out

Going back 30 years.
Amidst storms that tore through the sky, a series of massive earthquakes, and catastrophic anomalies that seemed like the end of the world, "that" suddenly appeared in the waters off Japan.
"The Ancient Capital Amatsuchi"
A mysterious cluster of ruins standing on a vast island, and an endless labyrinth stretching underground.
Spaces and grotesque creatures that defy all common sense.
That island, which brought enormous chaos to the world, has been under constant investigation at great sacrifice—yet even today, the outline of its full picture has never been revealed...
Before long, under the leadership of the United Nations, the island became independent as the "Yatsuba Republic." In areas secured as safe, ordinary people are now permitted to enter, and today it has become a world-famous tourist nation.
Located here is the Yatsuba Republic's only institution of higher education—Amatsuchi Academy.
Initially, the investigation of the underground labyrinth was led by the United Nations, but now most of the work is undertaken by a portion of the students who have received special education.
If the full picture of Amatsuchi is an "invisible end," then taking it slow is fine.
Anyway, let's set it aside for now and leave it to the next generation!
Or perhaps, elevating adventure in the ruins into entertainment (adjusting direction) would be nice too?
Wanting to make a name for themselves, wanting to touch the romance of the unknown, wanting to get rich overnight... and so on,
People with all sorts of motives have gathered at Amatsuchi Academy from around the world.
And then, another boy arrived here—not drawn by the mysteries of Amatsuchi, but visiting Yatsuba for a certain purpose. His name is Riku Ayase.
Will honor or mysterious secret treasures await him?
Beyond countless encounters lies an adventure tale full of danger and romance, overflowing with unknown traps and monsters.
What awaits him beyond the adventurer's days that began without warning...?