Submarine anchors - used when and where?

[bottomcrawler]

Last year, I saw a photo of a submarine from below, and the caption said that it showed the submarine's anchor!

It had never occurred to me before that subs have anchors, but apparently they do.

Now, when and where do subs anchor? Are they used routinely or just for emergencies?

I can't imagine that dropping and retreiving an anchor can ever be anywhere near silent.