Figure E13 YIELD

Through a rude tunnel, north once more we fare

Into a chamber perilous indeed:

Here, deep in Afric stone, a crystal rare,

The yellow francevillite, sees water bleed

And moderate the neutrons’ fatal seed

To start a chain-reaction underground

Two billion years before mankind will breed.

The hills of OKLO ferment without sound

Or outward sign, a grim Prometheus yet bound.