“It’s still too complicated!” Nina cried. “And it’s too arbitrary. The first plus the third divided by the second? It’s like setting a crossword puzzle but missing all the down clues!” She pointed the light back at the other walls. “The clue must be somewhere else, in the other text. I just have to find it.”

“Tick-tock, Doc,” said Chase, pointing at his watch. “Twenty-nine minutes.”

Nina knelt at one of the walls, scanning the light over the symbols. After a minute, she blew out her breath in frustration. “All of this is about the building of the city and the history of the people afterwards. I don’t see anything that relates to the puzzle at all.”

“There’s nothing about the people before they came here from Atlantis?” Kari asked.

“Not that I can see.” Nina hurried across the chamber to look at the text on the opposite wall. “This is more of the same. It’s almost like a ledger, a record of the tribe year by year. How many children were born, how many animals they had… There must be a couple of centuries of data here. But none of it has anything to do with the challenge!” She jerked an angry thumb at the symbols over the door.

“I just thought of something,” Chase said. “This thing’s a challenge of the mind, right? Well, what if it’s some sort of lateral thinking puzzle?”

“What do you mean?” asked Kari.

“This is obviously a door, right?” Chase stepped up to it. “We didn’t even think about just opening it.”

“Give it a try!” Nina told him.

Chase reached out and pushed the door. It remained completely still. He tried one side, then the other. Nothing happened. Just to be thorough, he also attempted to lift it, then pull it outwards from the wall. Still nothing.

“Bollocks!” he exclaimed, stepping back. “I really thought that might work.”

“So did somebody else,” Nina said, joining him. “Look! I just realized, the door’s not quite the same color as the rest of the chamber. It’s been carved from different rock. And there are marks on the stones around it-chisel marks, and crowbars. But none on the door itself. This is a newer door; the Indians have replaced it! Somebody didn’t want to solve the puzzle, so they just smashed the door open.”

“The Nazis?” Kari wondered.

“Sounds like their kind of approach,” said Chase. “They must have been able to persuade the Indians to let them bring more than just a flashlight inside.”

Kari nodded. “Probably at gunpoint.”

“Right. Problem is, we don’t have any crowbars. So we’ve got to do it the hard way.”

Nina hurried back to the carvings on the side wall. “I think we still can. These numbers, there’s something odd about them. Look.” She ran her finger along the lines of symbols. “You see? They’re arranged in groups of eight, at most. Never nine or ten. Eight here, eight here, eight here…”

“You think they could have been working in base eight?” asked Kari.

“It’s possible. They wouldn’t be the only ancient civilization to use it.”

“What’ve you found? What’s all this eight stuff?” Chase asked.

“I think we’ve been projecting our own biases onto the people who built this temple,” Nina said, excitement glinting in her eyes. “We assumed they were using base ten math, like we do.” She caught Chase’s questioning look. “Our numerical system is based around multiples of ten. Tens, hundreds, thousands…”

“Because we’ve got ten fingers, right. I did pass my GCSE maths,” he said. “Well, just about.”

“It’s a very common system,” Nina went on. “The ancient Greeks used it, the Romans, the Egyptians… It’s common because it’s literally right there in front of you.” She held up her fingers to demonstrate. “But it’s not the only system. The Sumerians used base sixty.”

“Sixty?” hooted Chase. “Who the hell would use that?”

Kari smiled. “You would. Every time you look at your watch. It’s the basis of our entire timekeeping system.”

“Oh, right.” Chase nodded sheepishly.

“There’ve been plenty of other bases used by ancient civilizations,” Nina continued. “The Mayans used base twenty, Bronze Age Europeans used base eight…” She snapped her head around to look at the symbols again. “Base eight! That’s it, it must be!”

“Why would anyone use eight?” Chase asked. In response, Kari held up her hands, fingers splayed-but with her thumbs tucked against her palms. “Oh, I get it-they used their thumbs to count on their fingers, but didn’t actually count the thumbs?”

“That’s the theory,” said Nina, searching through the inscriptions. “So instead of going one, ten, one hundred, the numbers actually go one, eight, sixty-four…” She rushed back to the door. “So the first column is single units, the second multiples of eight, then sixty-four, five hundred and twelve, four thousand and ninety-six and…”

“Thirty-two thousand, seven hundred and sixty-eight,” Kari finished.

“Right. So the number would be, let’s see… three single units, plus five units of eight, forty, plus sixty-four times seven…”

Chase made a pained noise. “I’ll let you two work all that out.”

Kari came up with the answer first. “Fifty thousand, six hundred and sixty-seven.”

“Okay,” said Nina. “You do the second number, I’ll do the third.” Another burst of mental arithmetic produced the answers: 36,695 and 14,452. “All right! So the first minus the second minus the third is…”

They both thought hard about it, Chase watching intently-only to see both their faces fall at almost the same moment. “What? What’s the answer?”

“It’s minus four hundred and eighty,” Nina told him despondently. “It can’t be base eight.”

“What about base nine?” asked Kari. “If decimal gives too large a result, and octal too small…”

“The answer would still be in the thousands. Shit!” Nina gave Chase a questioning look.

“Twenty-four minutes.”

“Damn it! We’re running out of time!” She angrily kicked the door. “What the hell are we missing?”

Chase crouched and rummaged through the lead balls, hoping there would be some hidden clue in the trough. There wasn’t. “What if we just take a best guess and put that many balls in the pan? There’s a chance we might get lucky.”

Nina touched her pendant. “That would need the biggest piece of luck in the world.”

“It’s all we’ve got. We can’t just give up-even if we go back through all the other challenges, the Indians’ll kill us as soon as we get outside. And Hugo and Agnaldo and the Prof.”

“If we get it wrong, we’ll be killed anyway,” Kari reminded him, pointing at the spikes suspended overhead.

“Maybe there’s some way we can pull the lever from outside the room…”

But Nina was no longer listening. Something else Chase had said was now foremost in her mind.

Back through all the other challenges…

That was what had been troubling her, gnawing at her mind. And now that she knew what it was…

“There’s another way through!” she burst out. “There has to be! The tribespeople maintain the temple, and the traps-they must, they need to be reset. And repaired.” She indicated the stone door. “But there’s no way the temple’s builders would have forced the very people who were supposed to be protecting it to go through the challenges every time they needed to go inside-one little mistake, and they’re dead! So there has to be some way for them to get through safely without running the gauntlet every time.”

“A back door?” asked Chase.

“Yes, like a service access, or even just some way to open the exit of each challenge without actually having to complete it.” Nina turned the light back to the chamber walls. “Maybe there’s a switch, or a loose block, some way to open the door.”

They hurriedly searched the walls of the chamber, fingertips brushing over the cold stone to feel for anything out of place. After a minute, Chase raised his voice. “Here!”