A crashed spaceship. As you do.
It was embedded quite deep in the ground.
And steam was still pouring out.
Then we heard a noise - music, and the sounds of people cheering - from the direction of the Mall (the wide, straight red-tarmacked road that runs from Buckingham Palace to Trafalgar Square). We hurried forward, and what did we see?
A giant mechanical walking elephant - heavier than Nelson's Column, and taller than Admiralty Arch - heading towards us, surrounded by a herd of marshalls carrying a circle of tape, which was itself surrounded by hoards of amazed people.
(Strictly speaking, although its legs were moving, it wasn't quite walking, because it had some little wheels underneath, but your eyes were so transfixed by the sight that you scarely noticed).
The beast was spraying water from its trunk across the crowd.
And it had a little house built on top, with people in, and more people on the house's flat topped roof.
Following it was a generator vehicle of some sort, and a trailer, upon which sat a band pumping out some exotic, but pleasing music.
It was, quite simply, one of the most awesome things I've ever seen in my entire life.
More details available from the BBC page for the event.