The result? We don't know, we're still living it. Every day people set out to destroy the world -- mostly through boosted use of fossil fuels -- and we don't know what's going to be left.

/fin

"The first atomic blast
Flashed through an empty city
River vanishing in steam
They had left the bomb there
The runes marked on its case
An offering
Perhaps the rock would be left,
Some said, imperishable
Or the secret name be freed
Or the world vanish"

In this imagined world nuclear explosions were invented not as weapons but as a sort of religious and scientific test. Or, really, the answer to an intolerable insult or not acceptable condition.

"The cryptographers
-- Impossible, they said --
The cryptographers
-- Impossible, they said --
Any message would have fallen to them
Codes crack, shatter
There must be none"

The two repeated lines aren't an error. They're a break, like the weather alert that interrupts conversation in "As you know, Katrina". The symbols on the rock were the name of God or maybe a fundamental observation of science or maybe they meant nothing.

"And a special guard
All facing in
Watched it, unceasingly
In case it might change again"

The people took the symbols used in their language from (naturally occurring?) symbols on the rock. But there was a passed down story that after they did this, something changed.

So the guard faces inward, towards the rock, to see if it ever changes again, or to keep it from changing through their continued observation.