a few thousand years, I'd say, since somehow a
remnant of the population managed to survive. This suggests that they
had some warning. Enough time passed for them to build the big structure
we noticed at the north pole, probably to store food, build underground
greenhouses and make sealed homes for a few families. Inside this giant
building the last of the Plutonian people kept going.

"Then came the moment when their planet fell into an orbit around our
Sun. I'd guess they emerged to find that the new Sun was too far away
ever to heat up Pluto again, or to permit the rebuilding of an
atmosphere. So they worked out a new scheme. This was to blow up the Sun
into a nova--make it a giant and thereby bring its heat all the way out
to Pluto--warming this world again, lighting it again, unfreezing its
gases and waters. So they set up the Sun-tap stations."

"That also accounts," added Haines, "for their limited number of
spaceships and their need for secret operations."

"Yes," said Burl, "but there are two things that don't fit in. What
happened to their moon--surely it would have gone along with Pluto since
it revolved around it? And second, why the thirty-year delay between the
first Sun-tap stations and the completion and operation of them?"

There was no answer to these questions yet. The three began the
morning's expedition.

As they neared the pole, they stayed close to the surface, for, any
moment, they expected to see the dumbbell ships that patrolled the sky
above it.

At last they set down the rocket plane on the edge of the polar plateau
and got out. Not more than a mile away, the black ramparts of the
building--a wall running miles across the horizon--rose hundreds of feet
into the sky.

Above it, they caught a flicker from the forest of masts and the glint
from a dumbbell ship. They moved silently forward, carrying the rocket
launcher on their backs and a small load of shells and several hand
bombs. These made heavy baggage, but the distance was not far, and the
purpose great.

Burl felt like an ant about to creep into a human house. But he
reflected that no ant ever had such dangerous intentions. An ant enters
a house to steal a crumb of food. But if an ant had intelligence and
evil intentions, it could cripple such a house.

Such was the situation for the three of them as they neared the
precipitous walls. On arrival, they found that entry would be easier
than they expected.

The Plutonian refuge