Where are Time Machine local backups/snapshots stored?
Under the new file system in High Sierra (Apple File System (APFS)), this it is different.  You can see what is considered the "local backups" via command line like this:

sudo tmutil listlocalsnapshots /


Ones that are listed as "(dateless)" will cause all sorts of problems:

com.apple.TimeMachine.2017-12-23-090037 (dataless)


so you can delete any backup by command line like this (deletes the one listed above, note only date used in reference):

sudo tmutil deletelocalsnapshots 2017-12-23-090037


Note that in my latest problem with macOS, the "bird" process was eating up egregious amounts of CPU. It was suggested that dumping backups that were damaged (damaged, who knew?) would help.