They seemed to use mechanisms for that, though - not Metalbending.
The domes appeared to have a "drawbridge" style series of chains and pulleys.
Moving on... Did Mako just fucking die?
http://imgur.com/a/z9R6O
KORRASAMI CONFIRED!!!
jk, I think he's fine. I HOPE they don't kill off a character like that without any confirmation. That would suck.