The ruin was flooded. It had been for centuries, and even after all that time, the water level had never decreased.
The local village warned their children not to stray too close to the ruin, though when asked why by outsiders, no one could say why the warning was given. They'd received it when they were children, and so had their parents, and grandparents, for as long as anyone could remember.
The children were warned to stay away.
Despite the warning, every adult in the village could remember going into the ruins as a teenager. It was wet, and cold, and smelled like something might have recently drowned. But the water was crystal clear, even in the deepest halls that led to some underground area of the ruin. You could see clearly all the way to the edge of your light, enough to see that the sloped hallways always turned to some deeper, hidden corner.
So the adults and teenagers knew the ruins, at least partially. But still they felt that their children and siblings wouldn't be safe there.
Outsiders suggested someone take some scuba gear and properly explore the underground chambers, but the locals never gave them access. They didn't remember the purpose for the ruin, but they felt that it wouldn't be good to let an outsider go that deep into it.
Some outsiders troed to sneak into the ruin anyway, but somehow somethung always stopped them.
Broken gear.
Dangerous wildlife.
Violent storms.
Local authorities catching them.
Injuries just as they reach the ruin, forcing them to turn back.
Something always came up.
So the ruin sat stagnate with water, a local mystery that no one seemed willing to solve.
For what good would come from drying out whatever was best left submerged?