Styx
The River Styx was once a river as its name suggested, its waters frothing with the lamentations of both living and dead. In those days the ferryman was enough to keep would-be intruders out, but as the years passed and Hades' grip on his realm grew ever tighter, it became clear that even a river circling the Underworld seven times over needed an upgrade.
And so, under Hades' watch, the shades stacked stones, bricks, and mortar until the Styx became a wall. Composed of iron and concrete, with razor wire at its apex and guard dogs placed at every station, it was much more effective at keeping intruders out. Hades was satisfied, but knew improvements could always be made, and the wall grew ever thicker and higher.
The Styx listened to the mortals' dismay at the creeping terror of death, and the shades' quiet resentment of their afterlives and endless work, but it was only enough to cause slight cracks, easily fixed. What finally caused the mighty Styx to split open, weeping, was a song of love and hope.