Spectacular phenomena of nature are a rare joy to witness.
Wikipedia hosts a fine write up on the migration of the Monarch Butterfly here which explains the phenomena, but that hardly describes seeing it live.
We would have missed it completely but for a chance encounter with a gentleman while waiting to disembark the ferry from Baja California. He was, I believe, an enthusiast, and had been observing the migration for some years whenever possible. This footage is from March, when the butterflies begin their journey back towards Canada and the Northern US.