Excerpt:

James slammed his book shut and put it carefully back in his bag, which was still stowed under the bunk. It had been a few hours since the bunker doors had closed - and James was getting hungry. He hadn't eaten since what might have passed for breakfast all those hours ago. But James didn't want the shame of being the first to raid the bunker's food supplies. Besides, if the raid was less than 18 hours, you had to pay for what you took. Greedy bastards in the government, he thought, smiling slightly. He'd sooner go a bit hungry for 19 hours. But then, he was the stereotypical penniless student - although, he thought, he probably had a few more pennies than most, who wasted theirs on drink. But not enough for him to want to waste them on a bit of tinned-something from the bunker stores. It was all provided to keep for a long time. In case it was needed. James turned over, and suddenly recoiled as extra lights built into the ceiling flickered on.
"What the hell-" His last words were drowned out as a siren cut through the bunker. It was unbearably loud due to the echoing of the bunker. James listened to it for a moment. That wasn't an air-raid siren. The air-raid siren was deeper. This was pitched so high it felt like it was making his ears bleed. Dave dropped down from the top bunk, and shouted at James.
"What's going on?!" He asked, pulling earphones out of his ears and looking around fearfully. James shrugged at him, and looked around in a panic. Someone at the entrance to the living quarters was shouting something. Suddenly, he took a megaphone given to him from a very pale-looking occupant of the bunker. The siren dipped for a second, and James ran a little closer to try and hear what the man was saying.
"PEOPLE!" He cried, turning the megaphone around so it faced each corner of the bunker in turn as he spoke. James strained to hear what he was saying over the new, strange, siren.
"I'M A POLICE OFFICER!" He shouted above the chaos that was people shouting to each other, clambering down off bunks, and the seemingly ever-present echoing siren that cut through the air like a bullet into everyone's ears.
"GET UNDER YOUR BUNKS! RIGHT NOW!" The message was passed quickly through the assembled occupants of the bunker, who looked at each other with renewed panic. What the hell was going on? Was there an enemy inside the bunker or something? But the man at the front of the bunker kept on shouting the same message, with renewed worry in every repetition.