Straddling the border between Croatia and Bosnia, Mount Plješevica rises from dense forest, its limestone ridges standing sentinel over the valleys below. But something man-made mars the mountain’s natural face, a clue to the silent secret it harbors. At its base lies a massive concrete entrance, three stories high, its distinctive geometry suggesting it once accommodated something far larger than any ordinary vehicle.Inside, a vast network of tunnels penetrates deep into the mountain’s core, the remnants of a strange underground city. Their walls still bear traces of strange markings that hint at a more organized past.In the deepest chambers, rusted blast doors weighing over 100 tons hang askew from their tracks. Behind them, cavernous halls stretch into darkness. Rows of mysterious mounting points line the floor, suggesting the regular placement of massive equipment, while another room reveals an array of Cold War-era technological debris.These are the remains of the enigmatic Objekat 505…