A classic case of redundant design, this motorway actually goes nowhere. It is in some respects verification of Einstein's Theory of General Relativity in that, if you drive along it for long enough, you return to your starting point.
It was designed to divert traffic from driving across London and lorries were banned in the metropolis during the day in an attempt to move them onto this road. This was a brilliant plan apart from one small but significant flaw - there wasn't enough room for the traffic that wanted to use it.
Add to this the activities of the Cone Fairy and the recipe for disaster was complete. However, some planner who is obviously some kind of lateral genius or an abuser of mind-altering substances came up with the idea of lowering the speed limit in order to increase the speed of traffic.
Against all logic, this worked.
There are those who say that, when looked at from above, the shape of the M25 is a demonic sigil. This of course is completely wrong - Satan's just not that clever.