A dank smell assailed Donna's nostrils and she reached a finger to the wet concrete wall. "Did you build this?"
"No," said Karanga, "the network of tunnels was already here. Some from the gold mining that began almost two centuries ago, and some from the railways that were built through the hills to carry the gold away."
Donna peered at the tiny green glowing dots that illumined their pathway. "What kind of lights are those?"
"Glowworms. Now hush, or they'll go out."
The Doctor and River continued their conversation unabated and Donna noticed the glowworms dimming as they passed by. She took care with her steps and paid attention to what they were saying.
"That war could have been avoided," said the Doctor. "So easily. If the populace had ousted a few greedy people from government...but isn't that always the way?"
"There would have been other greedy people to take their place." River's voice was chilly. "Believe me, I've seen more of that sort of behaviour than I like. Even in myself. There was the time I-"
"You what?" The Doctor turned his head to her as they strolled on.
"Oh...nothing." River fell silent.
The Doctor sighed. "All those races involved in the fight for that one strategic planet."
"What do you know about the nanobots?" River asked.
"Well, the fighting was so bad that most of the forces involved were either defeated or turned tail and ran home. It came down to two armies. The invading Veton, at a fraction of their initial strength but still formidable, and the Thrikeshaw, our friends here, who were defending their own planet."
"I know that part," said River. "Then the Veton released the nanobots and the Thrikeshaw managed to get their DNA ark away just in time, leaving the planet for the conquerors."
"Ah, but the conquerors had little joy in their victory." The Doctor's voice sounded sad and tired. "The nanobots were self-multiplying and their controllability failed. They destroyed their own creators' fleet, leaving the Veton homeworld without protection."
"So nobody got the planet in the end."
"Nope. It's just a mass of writhing nanobots from the star to the forcefield."
"That's terrible!" whispered Donna to Karanga. "It must have been so hard to give up your home."
Karanga wiggled her head. "It was not I myself, nor any of us here. We have only the telepathic memories passed down to us in the DNA. That is loss enough for anyone to bear. But we are changed now. This is our home."
Donna looked up and realised the hallway was emptying into a door on the left. River and The Doctor were already out of sight, and the birds continued to follow. She reached the opening and poked her head around, completely unprepared for what she would see there. "Oh, my giddy aunt!"
The soaring cavern appeared ancient, shored up with old timbers in places. But what took Donna's breath away were the shiny high-tech consoles that filled the room. Kiwis sat at screens and tapped on interfaces with their beaks.
Karanga brushed against Donna's exposed ankle and sent what could only be described as a telepathic smile. "Welcome to the Thrikeshaw nerve centre."