The thick layer of turbulent clouds that surrounded the Delphic Expanse, a by-product of the Sphere network, through which Enterprise NX-01 had to pass to begin its search for the Xindi.
The thermobaric clouds form the outer boundary of the Delphic Expanse, a region some two thousand light-years across. Ships entering the Expanse must first push through this dense, stormy layer, and few of those that do so are heard from again.
In the Enterprise episode The Expanse in 2153, after the Xindi attack on Earth, Archer takes the refitted NX-01 through the clouds, fighting off a Klingon Bird-of-Prey on the way. The Vulcans warn him with the last transmission of the Vaankara, whose crew went mad soon after crossing.
During the third season the crew learn that the clouds, like the spatial anomalies inside, are produced by the network of Spheres built by the trans-dimensional Sphere-Builders to reshape the region. When the Spheres are destroyed in Zero Hour in 2154, the thermobaric barrier disappears along with the anomalies and the Expanse returns to normal space.