1 Prof. Jones: The legendary Sinbad once ventured across a vast desert. But he was picked up by a giant eagle and carried far away to the coast.
2 Prof. Jones: He struggled to free himself from the claws, thinking to drop safely into the water below.
3 Prof. Jones: But then he spotted a giant flatfish waiting below, mouth wide to engulf him. He couldn’t decide which creature was more dangerous.
4 Monty: So...?
4 Prof. Jones: He was stuck between a roc and a hard plaice.

The idiom "stuck between a rock and a hard place" has an origin that was, to me, surprisingly recent when I did some research.

I knew that it was a very similar idiom to "between Scylla and Charybdis", referring to the legendary monster and monster/whirlpool between whom the hero Odysseus sails in Homer's Odyssey. According to Wikipedia, a phrase referring to Scylla and Charybdis as an idiom referring to escaping one hazard only to fall into another was first recorded in written usage in 1515, though of course presumably it was around in spoken language some time before then. Potentially even as far back as the Ancient Greeks.

Anyway, in my mind, "between a rock and a hard place" was essentially just a simple morph of this phrase. Scylla lived near a great rock, and Charybdis was the "hard place" on the other side of the channel. Without doing the research, I'd tacitly assumed that it too was of ancient provenance.

But no! According to The Phrase Finder, the idiom "between a rock and a hard place" dates to almost exactly 100 years ago. It was first recorded in written usage in 1921, in in the American Dialect Society's publication Dialect Notes V. It records that the phrase was in common usage in Arizona and sporadic usage in California, and related to recent disputes in the mining industry between workers and their overseers. It came about as a phrasing of the dilemma facing miners, of being stuck between a literal rock—the mine face they had to work on in underpaid and miserable conditions—and the "hard place" of unemployment.

Not wanting to trust a single source for this, I searched more widely, and found several other references to the same origin, but it looks like most, if not all, of them are merely copying the research done by Phrase Finder (as I am now also doing). So I hope they've been thorough.