This portal is viewed in a number of different lights by those who know of it. Some regard it as an amusing example of the Unity of Rings. Some consider it a blasphemy against Law or Chaos, or a sign of the supremacy of one or the other. Some just use it to cut down on a long trip.
See, there aren't many planes on the Great Ring that could be said to be farther apart than Mechanus and Limbo. Yet the Fractal Portal connects them, and in a way that suggests that ultimate Law and ultimate Chaos may not be that much different from one another.
From the Mechanus side, the Fractal Portal is located in an area of Mechanus where the gears get smaller and smaller, spinning faster and faster. They divide, subdivide, and subdivide again, interlocking up and down, left and right in intricate patterns of complex structure. A planewalker traveling along the gears soon finds that the "safe" area in the middle of the gears, where centrifugal force is negated by the nature of the plane, is too small to hold her. Fortunately, gravity is weaker as well, so that she can float along the increasingly intricate lattice by pushing off of the center of small gears. These shrink as she moves forward, to person-size, hand-sized, and even smaller, now whipping in their place with a blur too fast for the eye to discern. Touching off of a gear to advance imparts a bit of spin no matter how carefully she sets down her hand or foot, and finally she will trip and be thrown toward the region where the gears divide and shrink into dust and wind being spun impossibly fast. As the world turns about her, she finds herself in a very similar place... the soup of Limbo churning about her. She cannot simply pretend to trip; the key is that she must lose her orientation, unable to comprehend in its entirety the minute complexity of the gears.
From the Limbo side, the portal is referred to as the Ramsey Portal, and it must be approached by disciplined comprehension. All things arise in Limbo, it is said, and the theory of the Ramsey portal is that this includes even the stuff of order. A planewalker seeking Mechanus from Limbo via the Ramsey Portal must release any anarch's control over the chaos and dive into the soup with whatever protections she has, seeking a random eruption of metal. From near that spot she must find the next such eruption, and the next, and the next, following the trail through Limbo. Sometimes the trail will fork -- that is the way of chaos -- and following either fork will do, it seems, but follow one she must. Eventually, if her intent remains disciplined and firm, she will come to a place where the eruptions become more intense, splitting off more often and sending shards of metal spinning off into the roiling matter. Maintaining her focus, she must plow forward, into the heart of the intensity, and where it peaks she finds her foot falling on solid metal, the turmoil about her resolving to the whirring of a pattern of gears so intricate her mind cannot hold it all.
That chaos can arise from patterned division and repetition, disorder inherent within mathematical order, is blasphemy to some beings of Law. That within utter chaos there might lie any sort of unavoidable structure is repugnant to beings of chaos. And yet, the mathematics on both sides of the argument is inarguable (save by slaadi methods, which generally involve eating the mathematician). Further, the Fractal Portal certainly has significant utility, for those who don't worry about grand philosophizing.
A little more troublesome is the prospect that similar attachment points might exist on other planes, but the asymmetry between good and evil seems to militate against this. A portal linking Chronias to the uttermost depths of the Abyss? Hardly. It might be possible for extreme fanaticism in the cause of Good to create a path from Elysium to the Grey Waste, but what could take a traveler from the Waste to Thalassa? No, it seems most likely to observers who care to opine on the matter that Mechanus and Limbo are unique in holding such a peculiar point of attachment. Though that never stops speculators from looking.