Sometimes the Earth died in fire. Sometimes it was blown to pieces; other times it was ice that rose up against it. Jack might not remember all the variations, but he was old enough and Time Agent enough to recognise when the time line had been changed – after-effects of the Doctor's meddling perhaps, or his own; although it was just as likely to be something minor. Those famous metaphorical butterflies, and their hurricane wings; some tiny event in the life of an ordinary human, oblivious to the bigger picture and their brushstrokes upon it. The only thing he knew for sure was that, eventually, the Earth would always die. It was inevitable.
He was even there in the midst of it all once or twice – largely through gross miscalculation – trapped on that suddenly so fragile crust alongside what remained of the population. Saw the fiery hail rain down, or the sky torn apart, or the sun looming just that final inch too close to the atmosphere. Burned up only to wake again, in the cold deadness of a weirdly Earthless space. Sometimes he watched it from afar. Maybe it was an act of kindness on his part – like the death-watch at the bedside of an old friend, ready to hold a cooling hand in the final moments. The Earth was not the planet he had been born on, nor even the planet he had lived on the longest – with a life as long as his, all planets were home, at least occasionally. But the Earth, somehow, was special. It was where he had learned to be himself. It had been his prison, and also his cradle. It was where he had met the Doctor.
He stood now on the deck of a Kalgorian mine-ship, alongside a crew ready to gather up what minerals might be recovered from the approaching end. This time it was volcanoes – massive, super-powerful volcanoes, conjured up by tectonic activity gone wild. Humanity had moved on long ago, taking their 71st century Noah's Ark alongside them, away to populate a dozen distant worlds. The continents had shifted into new, extreme patterns, the sea levels risen high, the clouds turned broiling and black by volcanic effluvia. It was like looking at a stranger. Jack watched it anyway, as the fires and lava engulfed the surface, and the clouds engulfed the sky. As the juddering death throes of all that geothermal madness tore cracks no amount of lava could fill. As the planet of his forebears cracked and roared and, in the end, drifted away on tendrils of smouldering dust, that painted a last, fleeting signature across an indifferent void.