From the Great Buildings site, the Roman Patheon.
  This site offers a Quicktime video of the Pantheon.  When asked, click the "Test Drive" button to see it.
  Current construction technology continues to adapt the dome structure to suit various uses.
Think back to the diagram of the stable arch.  That construction worked because we applied compression at the bottom so that the walls would not kick outwards.

Now take the next step: In your mind, spin the simple arch around its highest point...what do you have?  A dome!