River led the entourage deeper into the room. Near the back they approached a structure that buzzed faintly, emanating a bluish glow from a small translucent sphere that hovered on top of a boxy device. The Doctor got down on hands and knees, for it was well below his eye level, and squinted into its centre.
A drip fell squarely on Donna's nose, and she clutched her chest for a moment. Then she looked up at the damp ceiling, and noted that all the computers had hoods to protect them.
"Easier to look at it here," said River, and unfolded a screen. "There's a camera in the base."
"Oh," said the Doctor, and pulled his glasses out of an inside pocket.
River turned on the screen and Donna watched, fascinated, as a multi-legged creature appeared. River pointed. "It's damaged, so that's why we were able to catch and isolate it." She tapped the image.
The Doctor peered through his glasses. "And why it isn't reproducing itself." He pointed at some shiny flecks that floated like dust motes. "I see it has managed to produce waste, though."
"Sparkly waste," remarked Donna to Karanga, because she felt she'd been quiet for too long.
Karanga chortled. "It's gold."
Donna started to laugh, but the Doctor nodded. "Certainly not the only way gold comes into being, but that is pure elemental Aurum."
"You have got to be kidding me." Donna gaped around at the serious faces. "But how do they do that?"
River sighed dramatically. "That's a complex question, and an answer for another day. We have weightier issues to deal with at the moment."
"Oh, yeah," said Donna under her breath. "I bet the universe needs saving, or something equally grave."
River shot her a look. Not a harsh or condescending look, as Donna might have expected in reply to her sarcasm, but a sympathetic smile that held depths of seriousness. There was more to her than met the eye.
While the Doctor and River examined the readouts, Karanga began to explain. "We only gained our sentience after these mines were depleted and closed down, and we thought they were ordinary gold mines. But since we found this bot, we became suspicious - that a colony of them may have left the gold behind. We can never know, because they're gone, and their waste is no different to any other gold."
The talkative bird fell into silence - thank goodness, Donna thought, it was like school all over again. She blinked at the glinting flakes that swirled inside the forcefield, barely visible to the naked eye, and slowly shook her head. "They poop gold."
The Doctor nodded. "The star system they're trapped in? By now it's got to be at least 50% gold. See why we have to keep it under wraps?"
"Half the space of a system turned to gold. Plenty of folks'd want a chunk of that," said Donna.
"And," said River, "no one could get at it without freeing the bots."
Donna frowned. "Is that what happened then? To let this one out?"
"Probably," sighed the Doctor, "but it won't be the only one if that's so."
"So what's the worst case scenario?" asked Donna. "How far would they be able to spread without a ship?" Immediately she realised the silliness of the question, but Karanga, ever the schoolteacher, was already answering.
"They can join together and build a ship, so that no place would be safe from them."
The Doctor straightened and rubbed his chin thoughtfully. "The real question is, how did this one get here? And are there any more?"
"Hold on, look here." River bent over the readouts. "It's been transmitting on an ultra-ultra high frequency."
"Ah, but is it doing it blindly, or does it know that there are others?" put in the Doctor.
River pressed her lips together for a moment. "We must assume that there are more, and they are coming."