I heard somewhere that the new Mac OS runs on top of Linux, or something like that, and that's why it's more stable now. I noticed that you can open a terminal and it looks like Linux. When did this start? What is actually underneath the Mac OS?
Nope, Still Good ol' Darwin :)
Essentially, it's more BSD based than Linux, but it also includes NeXTSTEP and Custom Apple code.
Mac OS X was a complete rewrite of Apple's operating system, and has (at its core) nothing to do with previous versions (such as Mac OS 9). So that is when the change happened.
It is not based on Linux, however, but on NeXTSTEP, an OS developed by Steve Job's company after he left Apple, that he brought back with him. NeXTSTEP is based on BSD. Both BSD and Linux belong to the unix-y family of operating systems, and they can look just the same to the casual user, especially since most command-line tools (such as the command shell itself) are available for both.
I heard somewhere that the new Mac OS runs on top of Linux, or something like that
When did this start?
Over nine years ago, 24 March 2001 (desktop version), there was a preliminary server version released in 1999.