What happens to a folder if I mount another filesystem on it?

The scenario is as follows:

After some time I reconfigured fstab because I decided that it would be better to mount another harddisk/filesystem on /var. The other day I (mistakable) started without this harddisk being present and I (accidentally) saw that the content of /var were the old files from the time before I switched to using this harddisk.

My question is now: Where are these (old) files while the new harddisk is mounted on /var??

Do they live in a miscelanous nirvana?