How To Mount And Use Multiple Drives

I must not even understand the basics of how mounting works in Linux.

Physical drive /dev/sda1 is mounted at /

I created a software RAID array (using mdadm) out of two other drives using mdadm, at /dev/md0, and mount it at /vol:

mount /dev/md0 /vol

Code:
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 9.9G 2.2G 7.3G 23% /
/dev/md0 200G 470M 200G 1% /vol

Now I start creating files in /vol/*

Then I unmount /dev/md0

I still see the files at /vol/*. They seem to be on /dev/sda1.

So how do I use the RAID array at /dev/md0 that I tried to mount at /vol?