Beneath the school, ten metres into the ground, the City Data Warehouse muttered quietly, uninterruptedly. Maja knew every cool corridor and the blue lights of the server racks. Over the summer, she took on duty as an intern for the Young Administrators programme, along with Kacper. Ms Fish, the supervisor, kept saying that data is like a river under the city. You have to keep an eye on it, clean out the twigs of errors and not trust the odd shortcut. That afternoon, Node Seven was spitting warnings, although all the sensors looked healthy. A folder named MAY_TOMORROW_08_17 flashed on the screen, with the date of tomorrow morning. Kacper snorted that it was a troll from the floor, but Maja froze at the keyboard. The timestamp had a negative delay, and the fans sounded like slow breathing. She opened the console, intending to check the track, when the clocks suddenly went back a minute. A communicator buzzed on her wrist and showed an unsigned message from a local transmitter. It read only: Don't open if the lights blink three times, in any order. The lights blinked twice and Kacper stopped jingling and took off his headphones. Mrs Fish went to the surface to calibrate the delivery drones; communications were poor. Maja scanned the folder, found a void of a mass of data, as if something was hiding the contents. She decided to isolate the file on a Slate service robot, as old as an underground map. Slate hummed, ejected the camera and created a sandbox, smelling of ozone and dust. At that moment the folder copied itself to all the dark links they hadn't known before. A countdown to auto-open appeared on the screen, starting at ten seconds. The lights flashed a third time, the steel door jingled and someone whispered names in the headphones. Maja looked at the counter: 00:00:09, 00:00:08, pulsing like a second, alien rhythm. Kacper jerked the emergency switch, but the plastic cracked as if it had managed to melt. A video without sound appeared on the communicator, and there was her own face. She was a day older, wearing the same sweatshirt, but eyes full of sparks. The screen flicked on, the timer jumped to 00:00:03, and her mouth began to form a word.