There was a tunnel under the town hall in Brzeziny that was not on any map. Lena held her torch high, as if leading a parade of ghosts, although she preferred not to think about it. She and Maciek belonged to the Urban Explorers Club, and Mrs Radecka had given them permission to take stock of the former pneumatic post office. The iron grating jawed as the keeper closed the entrance behind them and promised to wait by the stairs. Above them trams rumbled, but here it sounded like distant thunder. The air was cool and tasted of rust, and the walls glistened with a heavy dampness. Lena marked sections on the plan, comparing faded sketches with what really existed. Old porcelain insulators hung from the ceiling, shrouded in a thin web of grey dust. Maciek walked behind with a meter that beeped every now and then, even though it shouldn't. The signal on the phone disappeared as soon as he descended, but the torches held their shape, for the moment without a whimper. At the bend they came across something neither of them had anticipated. In the dust lay imprinted soles, too fresh to have belonged to the guards on their morning rounds. Next to it, someone had drawn an arrow in chalk that didn't match Mrs Radecka's plan. There was a beeping noise from the walkie-talkie and then a quiet, drawn-out word, as if a whisper: come back. 'Are you the one who said that? - Lena asked, glancing over her shoulder at Maciek. 'I didn't press the button, I swear,' he replied, and his meter suddenly went silent. Further down, under the vault, a pale light blinked, as if someone was checking the batteries and covering the torch. There was a draught coming from the tunnel, and the smell of ozone stung the nose, like before a storm. An arrow led to a steel door, which was absent from the plan. The hinges looked unhinged, but the padlock was new and a tad greasy from oil. Lena knelt down, touched the metal with her fingertip and withdrew her hand as the cold pierced her up to the shoulder blades. 'Someone comes down here more often than we do today,' she said in a whisper, and the echo responded with a longer, wet breath. Before they could decide whether to turn back for the guardian, the walkie-talkie came alive again without warning. They heard their own names, spoken in their voices, with only a slight delay, like a recording played on a loop. The lights flicked on and off for a split second, and when they came back on, Lena noticed something on the door. Underneath the padlock, with a long stroke of chalk, someone had added another arrow, clearly pointing deep into the tunnel. Next to it, three exact letters that looked like her initials glittered alarmingly.