I initially thought it was one of the boys - and scolded one of them appropriately. Then the apt really started rocking and I realized the earth was having a mild heart attack. Things were rattling and I kind of expected to hear something crash but no; having just lain down on the couch I was able to feel the shuddering for over a minute after the main roller.

Sometimes, the air you 'let in'matters less than the air you 'let out'.