What happens if you throw your anchor overboard and there isn't enough chain to reach the bottom? Does the anchor haul you down with it?
Not A Laughing Matter
This sounds a little bit like something that would happen in a cartoon. But for sailors it's no laughing matter. It's unlikely the whole ship would be pulled under, but there could very well be damage to the hull caused by not having enough chain, including a sudden hole where the anchor used to be.
That's why the nautical rule of thumb is to use three times as much chain as is needed to hit bottom. That extra chain piles up around the anchor and provides added stability.
Still, what if you don't know how deep the water is? Or what if you just make a mistake?
The navy has prepared for this by building what is called a "weak link" into their anchor chains. The weak link is the last link in the chain, and while it's strong enough to hold together in general, it's weak enough to break if the whole weight of the anchor and chain is put on it.
The Weakest Link
The weak link is there as a safety valve. Because it's the very last link, under normal circumstances it never gets played out at all; you always want to have some chain left on the ship.
But if the anchor is still zooming down full speed at the point where the entire chain runs out, the abrupt jerk will cause that link to snap.
Without the safety valve of the weak link, sailors might routinely drop more than their anchors.