The Martian Way and Other Stories
The Deep Chapter One

In the end, any particular planet must die. It may be a quick death as its sun explodes. It may be a slow death, as its sun sinks into decay and its oceans lock in ice. In the latter case, at least, intelligent life has a chance of survival.
The direction of survival may be outward into space, to a planet closer to the cooling sun or to a planet of another sun altogether. This particular avenue is closed if the planet is unfortunate enough to be the only significant body rotating about its primary and if, at the time, no other star is within half a thousand light-years.
The direction of survival may be inward, into the crust of the planet. That is always available. A new home can be built underground and the heat of the planet's core can be tapped for energy. Thousands of years may be necessary for the task, but a dying sun cools slowly.
But planetary warmth dies, too, with time. Burrows must be dug deeper and deeper until the planet is dead through and through.
The time was coming.
On the surface of the planet, wisps of neon blew listlessly, barely able to stir the pools of oxygen that collected in the lowlands. Occasionally, during the long day, the crusted sun would flare briefly into a dull red glow and the oxygen pools would bubble a little.
During the long night, a blue-white oxygen frost formed over the pools and on the bare rock, a neon dew formed.
Eight hundred miles below the surface, a last bubble of warmth and life existed.