Why do we say "Like rats deserting a sinking ship"?

Meaning:

When facing a great crisis most will turn and run.

Background:

Rats have been said to be the first to sense an impending disaster, such as a sinking ship or a gas leak in a mine - so if rats are seen leaving it's a good idea to follow!

Early variants of this expression go back all the way to Pliny The Elder's Natural History (77 AD): 'When a building is about to fall down, all the mice desert it'.

It also appears in a form closer to the modern usage in Shakespeare's The Tempest, Act I, Scene II (1610):

Prospero: In few, they hurried us aboard a bark,
Bore us some leagues to sea; where they prepar'd
A rotten carcass of a boat, not rigg'd,
Nor tackle, sail, nor mast; the very rats
Instinctively had quit it.

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