Carrying an extra engine on the 747 to Singapore

One cool thing about the flight to Singapore...  Our plane had an extra engine tucked away under the left wing!  I couldn't get a picture, but it was there.  I could see it from the lower deck.  It slowed us down just a bit the pilot said.  It is quite unusual, so I guess there was some emergency that required them to transport it in a hurry, but he said the aircraft are equipped intentionally to allow this when necessary.  The flight attendant told me in 21 years of service she has never seen it done though!  So kind of cool...