PONDER OPENED HIS EYES and looked up into a face out of time. A mug of tea was thrust towards him. It had a banana stuck in it. 'Ah ... Librarian,' said Ponder weakly, taking the cup. He drank, stabbing himself harmlessly in the left eye. The Librarian thought that practically everything could be improved by the addition of soft fruit, but apart from that he was a kindly soul, always ready with a helping hand and a banana.

The wizards had put Ponder to sleep on a bench in the store­room. Dusty items of magical gear were stacked from floor to ceiling. Most of it was broken, and all of it was covered in dust.

Ponder sat up and yawned.

'What time is it?'

'Ook.'

'Gosh, that late?'

As the warm clouds of sleep ebbed, it dawned on Ponder that he had left the Project entirely in the hands of the senior faculty. The Librarian was impressed at how long the door kept swinging.

Most of the main laboratory was empty, except for the pool of light around the Project.

The Dean's voice said, 'Mappin Winterley ... that's a nice name?'

'Shutup.'

'Owen Houseworthy?'

'Shutup.'

'William.'

'Shut up, Dean. That's not funny. It never was funny.' This was the voice of the Archchancellor.

'Just as you say, Gertrude.'

Ponder advanced towards the glowing Project.

'Ah, Ponder,' said the Senior Wrangler, stepping in front of it hurriedly. 'Good to see you looking so…'

'You've been ... doing things, haven't you,' said Ponder, trying to see around him.

'I'msure everything can be mended,' said the Lecturer in Recent Runes.

'And it's still nearly circular,' said the Dean, 'Just ask Charlie Grinder here. His name's definitely not Mustrum Ridcully, I know that.'

'I'm warning you, Dean...'

'What have you done?'

Ponder looked at his globe. It was certainly warmer now, and also rather less globular. There were livid red wounds across one side, and the other hemisphere was mainly one big fiery crater. It was spinning gently, wobbling as it did so.

'We've saved most of the bits,' said the Senior Wrangler, watch­ing him hopefully

'What did you do?'

'We were only trying to be helpful,' said the Dean. 'Gertrude here suggested we make a sun, and...'

'Dean?' said Ridcully

'Yes, Archchancellor?'

'I would just like to point out, Dean, that it was not a very funny joke to begin with. It was a pathetic attempt, Dean, at dragging a sad laugh out of a simple figure of speech. Only four-year-olds and people with a serious humour deficiency keep on and on about it. I just wanted to bring this out into the open, Dean, calmly and in a spirit of reconciliation, for your own good, in the hope that you may be made well. We are all here for you, although I can't imagine what you are here for.' Ridcully turned to the horrified Ponder. 'We made a sun...'

‘...some suns...' muttered the Dean.

‘...some suns, yes, but ... well, this "falling in circles" business is very difficult, isn't it? Very hard to get the hang of.'

'You crashed a sun into my world?' said Ponder.

'Some suns,' said Ridcully.

'Mine bounced off,' said the Dean.

'And created this rather embarrassingly large hole here,' said the Archchancellor. 'And incidentally knocked a huge lump out of the place.'

'But at least bits of my sun burned for a long time,' said the Dean.

'Yes, but inside the world. That doesn't count.' Ridcully sighed. 'Yet your machine, Mister Stibbons, says a sun sixty miles across won't work. And that's ridiculous.'

Ponder stared hollow-eyed at his world, wobbling around like a crippled duck.

'There's no narrativium,' he said dully. 'It doesn't know what size a sun should be.'

'Ook,' said the Librarian.

'Oh dear,' said Ridcully. 'Who let him in here?'

The Librarian was informally banned from the High Energy Magic building, owing to his inherent tendency to check on what things were by tasting them. This worked very well in the Library, where taste had become a precision reference system, but was less useful in a room occasionally containing bus bars throbbing with several thousand thaums. The ban was informal, of course, because anyone capable of pulling the dooknob right through an oak door can obviously go where he likes.

The orangutan knuckled over to the dome and tasted it. The wizards tensed as delicate black fingers twiddled the knobs of the omniscope, bringing into focus the furnace that had exploded yes­terday. It was a tiny point of light now, surrounded by coruscating streamers of glowing gas.

The focus moved in to the glowing ember.

'Still too big,' said Ridcully. 'Nice try, old chap.'

The Librarian turned towards him, the light of the explosion moving across his face, and Ponder held his breath.

It came out in a rush. 'Someone give me a light!'

The globes on his desk rolled off and bounced on the floor as he tried to grab one. He held it as the Senior Wrangler obligingly lit a match, and waggled it this way and that. 'It'll work!' 'Jolly good!' said Ridcully. 'What will?'

'Days and nights!' said Ponder. 'Seasons, too, if we do it right! Well done, sir! I'm not sure about the wobble, but you might have got it just right!'

'That's the kind of thing we do,' said Ridcully, beaming. 'We're the chaps for getting things right, sure enough. What things did we get right this time?'

'The spin!'

'That was my sun that did that,' the Dean pointed out, smugly.

Ponder was almost dancing. And then, suddenly, he looked grave.

'But it all depends on fooling people down there,' he said. 'And there isn't anyone down there ,.. HEX?'

There was a mechanical rattle as HEX paid attention.

+++Yes? +++

'Is there any way we can get onto the world?'

+++ Nothing Physical May Enter The Project +++

'I want someone down there to observe things from the surface.'

+++ That Is Possible. Virtually Possible +++

'Virtually?'

+++ But You Will Need A Volunteer. Someone To Fool +++

'This is Unseen University,' said the Archchancellor 'That should present no problem.'

WE DON'T KNOW IF THE EARTH IS A TYPICAL PLANET. We don't know how common 'aqueous' planets with oceans and continents and atmospheres are. In our solar system, Earth is the only one. And we'd better be careful about phrases like 'earthlike planet', because for about half of Earth's history it has not been the familiar blue-green planet that we see in satellite photos, with its oxygen atmosphere, white clouds, and everything else that we are used to. In order to get an earthlike planet, in today's sense, you have to start with an unearthlike planet and wait a few billion years. And what you get is quite different from what, only a few decades ago, we thought the Earth was like.

We thought it was a very stable place, that if you could go back to the time when the oceans and continents first separated out, they'd have been in the same places they are now. And we thought that the interior of the Earth was pretty simple. We were wrong.

We know a lot about the surface of the Earth, but we still know much less about what's inside it. We can study the surface by going there, which is usually fairly easy, unless we want to look at the top of Everest. We can also penetrate the ocean depths using vehicles that can protect frail humans against the huge pressures of the deep seas, and we can dig holes down into the ground and send people down those too. We can get further information about the top few miles of the Earth's crust by drilling, but that's just a thin skin, comparatively speaking. We have to infer what it's like deeper down from indirect observations, of which the most important are shock-waves emitted by earthquakes, laboratory experiments, and theory. The surface of our planet generally seems fairly placid, apart from weather and the sometimes severe effects of the seasons, but there are plenty of volcanoes and earthquakes to remind us that not so far below our feet it's a lot less hospitable. Volcanoes form where the molten rocks inside the Earth well up to the surface, often accompanied by massive clouds of gas or ash, all of it emerging under high pressure. In 1980 Mount St Helens in Washington State, USA blew up like a pressure-cooker whose lid had been tied down, and about half of a large mountain simply disappeared. Earthquakes happen when the Earth's crustal rocks slide past each other along deep cracks. Later we'll see what drives these two things, but they need to be put into perspective: despite occasional disasters, the surface of the Earth has been sufficiently hospitable for life to have evolved and survived for several billion years.