In Blue Highways, William Least Heat-Moon describes a system for identifying high-quality American cafés:

“There is one almost infallible way to find honest food at just prices in blue-highway America: count the wall calendars in a cafe.

No calendar: Same as an interstate pit stop.
One calendar: Preprocessed food assembled in New Jersey.
Two calendars: Only if fish trophies present.
Three calendars: Can’t miss on the farm-boy breakfasts.
Four calendars: Try the ho-made pie too.
Five calendars: Keep it under your hat, or they’ll franchise.

One time I found a six-calendar cafe in the Ozarks, which served fried chicken, peach pie, and chocolate malts, that left me searching for another ever since. I’ve never seen a seven-calendar place. But old-time travelers - road men in a day when cars had running boards and lunchroom windows said AIR COOLED in blue letters with icicles dripping from the tops - those travelers have told me the golden legends of seven-calendar cafes.”

On safari, we discovered a similar system for identifying significant sights: count the vehicles in the place you're racing toward.

One or two vehicles: antelopes or birds.
Three or four vehicles: one or two predators of some kind.
Six vehicles: a pride of lions.

And so, as we made our way along the river and saw the sight below on the far side, my curiosity was piqued. What sight would draw twenty-plus vehicles to a single place?

Many Mystery Machines massed in Maasai Mara.

The answer? The blue wildebeest (connochaetes taurinus).

Now, a single wildebeest is not much of a sight - just another member of the 'wastebasket taxon' of antelopes. But there are one and a half million wildebeest in the region, and they migrate around the Serengeti and Maasai Mara ecosystems in massive herds, often accompanied by herds of zebras and gazelles, seeking fresh grass to consume. Sometimes, this search will force a herd to cross the Mara River. This crossing is what the spectators were waiting to see.

The herd was gathered. But would it cross the river? To witness a crossing, you must be very patient or very lucky. As Daryl and Sharna Balfour explain in their their article about the wildebeest migration:

“Wildebeest arrive at the Mara River in their tens of thousands, and gather waiting to cross. For days their numbers can be building up and anticipation grows but many times, for no apparent reason, they turn and wander away from the water’s edge. Eventually the wildebeest will choose a crossing point, something that can vary from year to year and cannot be predicted with any accuracy. Usually the chosen point will be a fairly placid stretch of water without too much predator-concealing vegetation in the far side, although occasionally they will choose seemingly suicidal places and drown in their hundreds. Perhaps, once again, this is because crossing places are genetically imprinted in the minds of the animals.”

On this day, were were in luck. After a false start further along the Mara that left a single zebra alone on the far bank, the herd began to pour down into the river before us, a seemingly endless, roiling ribbon of muscle, bone, and dust.

Here is a six-second video of the crossing, which took the herd more than ten minutes to complete.

When the crossing was complete, I asked George to estimate the size of the herd. He first said "five to ten thousand", but then upped his guess to "ten to fifteen thousand", so I'll say ten thousand. And if you ask me what was the highlight of my safari, I'll say it was this: the crossing of the ten thousand.