Suppose you use ZFS and have pool/foo mounted at /foo.
zfs create -o mountpoint=/foo pool/foo
Suppose you’ve created a snapshot:
zfs snapshot pool/foo@bar
Now they tell you that you can access it inside .zfs/snapshot directory. However, if you go into /foo and type ls -la (option ‘a’ means you want to see files and directories beginning with a dot), you don’t see the .zfs directory. You’ll see ‘.’ and ‘..’ and all others, but not ‘.zfs’. However, an innocent ‘cd .zfs’ or ‘cd /foo/.zfs’ will get you there. A truly hidden directory indeed.
What was it called at esr’s The Art of Unix Programming? The Principle of Least Surprise?