That literary heritage ranged high and low, and included hymns and omens, but also, very old drinking songs. As in the Maya world, the link between writing and power was advertised through monumental inscriptions. Nabu-kusurshu's tablets were sustained and protected by an entire culture.

But there was, perhaps, also an element of individual choice. Nabu-kusurshu appears to have taken pride in his writing, and taken care to perfect it, given how exceptionally neat it was.

Crisostomo is scouring the world's museums for more of Nabu-kusurshu's tablets, of which about 24 have been found. He has studied every detail of the brewer's handwriting, from how he shaped his signs to how he spaced his lines. "It's things like that where you start to really feel like you know these people."

Despite his own love for written language, Crisostomo says his message for the future would probably be an image so that "it could transcend the need for language", and avoid the pitfalls of decipherment.

It appears, then, that a good rule of thumb is to make your message to the future either gargantuan enough that it can't be ignored, or so small that it slips through history almost unnoticed, perhaps protected by its low profile. A visual or contextual cue seems to help, be it by adding a picture, or placing it somewhere relevant to its meaning like a temple or monument. And the scholars appeared to find it obvious that it was better to use an existing language, than try to make up an artificial, future-proof one. After all, real languages have cultures to love and support them, providing future decipherers with a wealth of clues and meaning.

In fact, cuneiform is experiencing a renaissance these days, as a young generation of Iraqis learn and experiment with it. A similar spirit is infusing the Maya hieroglyphs with new life. Native Maya speakers use it to make art, and put up new stelae to commemorate important events.

That human connection and fellowship, across vast stretches of time, perhaps forms the final step for an immortal message. As much effort as we may put into it, we can only trust that at the other end of the line, there'll be another person hearing our faint voice, and caring enough to listen.

Crisostomo often remembers this when he works on ancient tablets, some marked by thumb-prints of long-dead scribes. "Sometimes you'll sit there and you put your thumb right in that same space, and you think, 'OK, maybe this person was holding this tablet just like this, 4,000 years ago, and they're holding it and they're writing it, and I'm sitting here, reading what they wrote.'"

* Sophie Hardach is the author of Languages Are Good For Us, a book about strange and wonderful ways in which humans have used languages throughout history.

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The messages that survived civilisation's collapse - BBC