Randall Munroe's xkcd cartoon site is a hit with programmers and techie types alike. His latest creation, however, is an enigma, a mystery and a piece of performance art.

If you go to last week's xkcd cartoon - number 1190 (March 25th, 2013) - then you will see a silhouette of sand castles being built. The mouse hover over text just reads "Wait for it".

If anything is any different when you visit then this is only to be expected. Xkcd enthusiasts familiar with past experiments would immediately suspect that this fairly uninteresting cartoon has something more interesting lurking below the surface - and it does.

If you wait for a while you will see the picture change. Yes, you are watching a very slow "movie". At first it updated every 30 minutes, but at the time of writing it takes an hour. It might stop at some point in the future, loop or just go on forever. It is the open-endedness of it that gets to some viewers. Now the "Wait for it" hover text snaps into better focus and at this point we could descend into existential twaddle, sorry philosophy, but...

In side the page is a script and it uses simple Ajax to download a new image every so often. The script is minified and so not easy to follow, but there are a number of programmers taking the time to figure it out. The request redirects server side to a new image and it is difficult or impossible to get at the future images (they might not have been created yet) but if you know the links you can get the previous images.  This has resulted in a number of sites putting the frames together to produce either an animated gif or a pseudo video that downloads each frame in turn. 

So far the two characters have sat on the beach (the sands of time?), gone for a swim, scratched sand out of their hair, built and knocked down really nice sand castles using a trebuchet. Recently the "action" has shifted to just one big tower - any guesses as to where it is all going?

Is this a cartoon, art, or what? I've seen installations with a lot less content or appeal.

A large part of the xkcd fan base has decided that whatever it is they want to be part of it. There is even a wiki dedicated to discussing it - http://xkcd-time.wikia.com/wiki/XKCD_Time_Wiki.

One final question - how will we know when it has finished?