The Mirage Volcano is an attraction outside of the Mirage Hotel. It goes off about ever 15 minutes. It is great fun.
It is crowded and if you don't want another person shoulder to shoulder with you skip it, but if you can handle that, you will enjoy it and it is worth the 15 minute wait.
It is free (YAY) and it explodes like a volcano. It may sound silly, but it is great fun.
Enjoy.