It looked very different from the simulator. The red sand of Mars, creaking under magnetic boots, stretched all the way to the horizon, and the thin atmosphere painted the sky with a pale orange light. For Maja, Peter and Lena - participants in the school's first interplanetary exchange - every second seemed unreal. They stood outside the Habitat X3 dome in light suits, gazing up at the darkening sky. A robot guide, called 'Titus' by everyone, was leading the group to the communications station. - 'Did you know that tonight Mars will be the closest to Phobos in a hundred years? - Lena said, looking at the messages on the hologram watch. - And wasn't it on Phobos that someone once discovered strange tunnels? - asked Peter, feigning indifference, although his eyes were shining with excitement. The robot stopped just in front of a low, grey structure. Titus had a panel displaying a simple smile animation that flashed as he activated the entrance door. - Time for a hands-on activity, young explorers. 'Your task will be to intercept and decipher radio signals from around Mars,' he said in a mechanical but strangely warm voice. - Maybe you'll hear satellite conversations ... Or something more exotic. Inside the station there was semi-darkness. The walls flicked with scattered emergency lights, and an air filter hummed quietly in the corner. Maja sat down in front of the receiving panel and swiped her finger across the screen. - 'There's a lot of noise,' she muttered. - 'But... wait, it sounds like a repeating pattern. Listen to this! A series of strange, interrupted sounds came from the speaker: beep-beep-beep-beep.... long pause... beep... beep-beep. Lena looked at Peter with concern. - 'I don't think this is a signal from any known satellite. Suddenly the panel flashed red and unknown characters appeared on the screen. Everything froze. Even Titus stopped moving, as if the received message had put him into a state of suspension. - 'See this,' Maja said in a whisper, pointing to the rapidly changing images on the monitor. - It looks like a map ... Leading somewhere beneath the surface of Phobos. Piotr felt his heart beating faster and faster. The group exchanged communicative glances. They knew they had to make a decision - and immediately. At that moment, the communications station shook and a brief message appeared on the main screen in a language none of them understood. The station door slammed shut with a clatter, and an unnatural, low sound resembling distant thunder rang in the background. A shadow ran across the Martian dust, as if someone - or something - was approaching the station at breakneck speed. Maja, Peter and Lena froze, staring at the shifting signs on the screen. Titus suddenly came to life again and turned his digital eyes to them: - You must make a decision. Immediately. A bright streak of light appeared behind the armoured window, aimed straight at them....