"These marks are simpler than the ones on the southern side," she said, standing up. "They might be older, or they might simply serve a different purpose. The short lines point toward the ice cap, possibly indicating the direction of the signal source."
That afternoon, they continued west along the edge of the ice cap. Harry checked his perceptions at several locations and found that the northern signal's frequency and amplitude were uniform throughout the area, though its encoding was slightly clearer toward the west.
He told Isabelle what he had sensed, and she marked the approximate center of the signal on the map.
The next morning, they turned toward the western side. The western marker lay near the entrance to a valley on the western edge of Vatnajökull Glacier, where the lava fields gradually gave way to moraine ridges and a river valley formed by glacial meltwater.
Helga led them upstream along the river valley for a while, then stopped near its mouth.
"The western marker is right here. Survey records from the 1950s mentioned intermittent signals here, but their periodicity was weak, so no one paid much attention at the time."
Harry crouched on a moraine ridge near the valley mouth and pressed his palm against the gravel.
Vibrations rose from deep beneath the gravel. Their frequency was lower than on the northern side, their cycle longer, and their amplitude extremely weak. Just as the records had said, they were highly intermittent: the signal appeared for a few seconds, vanished for ten-odd seconds, then appeared again for a few seconds.
These intermittent pulses were unlike the continuous vibrations at every previous node.
"It's highly intermittent," he told Isabelle as he stood up. "The signal appears for a few seconds, disappears for ten-odd seconds, then returns. Its frequency is