The view behind the porthole of the ARC Polaris ship was unearthly - an ocean of shadows and lights, the dance of the aurora borealis around the huge Jupiter, and in the distance, an inconspicuous ice globe. Lidia held her breath as she watched Kallisto approach. - 'I almost can't believe we're here,' she whispered, checking the communicator on her spacesuit for the hundredth time. Her friend Tom, sitting opposite her, shrugged his shoulders and tried to remain calm, but his fingers clenched nervously on the armrest. Next to them, in the pilot's seat, Andrei was programming the final landing commands. The Apex-2's automatic system was silent, so all three felt the weight of responsibility more than ever. - 'Recall our mission,' Andrei said, without taking his eyes off the panel. - 'Main objective: to explore the subsurface ocean. We expect a safe landing, collect samples and return. Tom struggled to hold back a smile. Of course, the official version was important. But who hasn't dreamt of discovering something that even the scientists at the command centre hadn't anticipated? - Will the sound sensors really come in handy? - Lidia asked. - Speaking to ice seems a bit... strange. - If there's an ocean under the ice, there might be some life forms too. 'NASA always says: be ready for anything,' stated Tom, quoting his favourite astronaut handbook. The ARC Polaris braked and for a moment all that could be heard in the cockpit was the quiet hum of the life support systems. The ship touched the surface of Kallisto softly. - Landing point: gamma sector - announced Andrei. - Pressure: stable. Ice cover: 18 metres to the ocean. As they put on their suits, Lidia felt a slight shudder - whether from excitement or fear, she wasn't sure. They opened the hatch and descended the slippery ramp to an ice desert illuminated by the ship's searchlights. Closer to Jupiter, the surface seemed unusually brittle, and in the distance, where the geolocation camera indicated an anomaly, the ice took on a deep blue colour. - It was here... - Tom knelt down and applied the microprobe to the ice. Signals were transmitted to the tablets. Everyone froze, staring at the screen. Was something vibrating under the ice? Suddenly their helmets began to pick up an unfamiliar sound signal. There were strange pulsations, as if someone - or something - was trying to respond to their presence. Earth's dawn was hundreds of millions of kilometres away here, and the three teenagers were faced with something that could not be explained by textbooks. Andrei looked at a concerned Lidia and Tom. The distorted, deep tone came from the speakers again, and the ice trembled beneath their feet. They all held their breath as the thin line of the crack began to spread slowly towards their feet....