t it, he knew it was because Gail would be in the plane.

When he headed back toward the main building one of the geophysics gang
beckoned to him. He followed to the small, far-spaced hut--now
snow-buried to its eaves--in which the seismograph ticked away to
itself.

"I think I'm going crazy," said the geophysics man. "Did you ever hear
of a ground-shock starting inside out?"

He pointed to the graph-paper that fed very, very slowly past the
seismograph's pens. The recording did look odd.

"If you put your hand just under the surface of the water in a bathtub,"
said the geophysics man harassedly, "and jerk it downward, you get a
hollow that spreads out with a wave behind it. It's the exact opposite
of dropping a pebble into water, which makes a wave that spreads out
with a hollow--a trough--behind it. But except for that one way of
making it, all waves--absolutely all wave-systems--start out with a
crest and a trough behind it. Everywhere, all the time, unless you do
what I said in a bathtub."

"I'm a shower man, myself," observed Soames. "But go on."

"This," said the geophysics man bitterly, "is like a bathtub wave. See?
The ground was jerked away, and then pushed back. Normal shock-waves
push away and then spring back! An ice-crack, a rock-slide, an explosion
of any sort, all of them make the same kind of waves! All have
compression phases, then rarefaction phases, then compression phases,
and so on. What--" his voice was plaintive--"what in hell is this?"

"Are you saying," Soames asked after a moment, "that ordinary
earth-tremors record like explosion-waves, but that you'd have to have
an implosion to make a record like this?"

"Sure!" said the geophysics man. "But how can you have an implosion that
will make an earth-shock? I'm going to have to take this whole damned
wabble-bucket apart to find out what's the matter with it! But there's
nothing the matter! It registered what it got! But what did it get?"

"An implosion," said Soames. "And if you have trouble imagining that,
I'm right there with you."

He went back to the main building to get Gail and Captain Moggs. They
went out to the 'copter hangar together.

"I've talked to the radar and loran operator," said Soames. "I explained
that you wanted to see some crevasses from the air, and I'd be wandering
around looking for them on the way to the rookery. He will check on us
every fifteen minutes, anyhow."

* * * * *