Lena studied cartography and worked nights in the harbour library, where maps smelled of salt and history. That evening she brought a roll of tattered parchment from the storeroom, wrapped in string and dried rusty ribbon. In the margin, someone had drawn a makeshift lantern and annotated it: When the shadow of the luna touches the lens, you go lower. The words sounded like a joke, but the parchment had salt stains and the edges were drenched from an old fire. - Do you know what this could mean? - She asked Tymek, a colleague from the sailing section, glancing at the draft of months. - As for the lens, the Secondary Lighthouse has an old Fresnel, long since deactivated and supposedly unsafe. Before sanity set in, they packed up their torches, Tymek's grandfather's compass, and set off towards the darkened breakwater. The wind carried the scent of seaweed, and seagulls darted overhead as the tide evened its breath with the city. The lighthouse stood at the edge of the headland, rusted and chipped, but the door gave way under their concerted, careful pressure. The staircase wound tightly upwards, and someone had carved a star grid on the handrail, consistent with the map from the library. - 'This isn't a coincidence,' muttered Lena, touching the steel rivet under which the almost invisible mark was stuck. A crow cawed from a broken window, as if to warn, but they both moved on, curiosity pushing them higher. The lantern room was cool, and the great Fresnel lens rested crookedly, cracked like a dried-up riverbed. On the floor lay a brass disk engraved with the phases of the moon and notches to match the compass. As the twilight lit up orange, Lena adjusted the disk and Tim held the needle until the mechanism clicked. Gears somewhere beneath them moved, the floor vibrated, and with a hiss a panel lifted, revealing a stone staircase down. They descended slowly, illuminating the steps, smelling tar and machine oil, as if the lantern breathed beneath a shell of silence. Tide streaks ran along the walls, and in white chalk someone had drawn arrows that turned back in unexpected places. Above a shabby hatchway was the phrase: Don't go back if you don't know where you've been; a compass trembled in his hand. In front of them stood an iron door with a porthole, from behind which a light pulsed; Lena gripped the wheel and a shadow moved behind the glass. The wheel baked, then let go and groaned as Lena and Timek unanimously turned it one turn. Somewhere above them a lens squeaked, a barely-there shadow of glow shone in the lantern, as if someone had smoothed out the darkness. A chill and the scent of the depths wafted through the gap, and the walls responded with a tapping, short, anticipatory, precise as a signal. - 'Do you hear that? - whispered Tymek as something knocked again, closer, and the darkness behind the door moved more clearly.