Rima Sirsalis is in an unusual location for a rille. Most rilles are near the edge of maria, or inside them. Rima Sirsalis cuts across highlands almost exclusively, and seems to go in places where such a formation would be impossible. It starts at the edge of Oceanus Procellarum near the modest Sirsalis Crater from which it gets its name and proceeds to directly away from the mare, eventually ending up among the cracks in the floor of Darwin Crater. Along the way it passes through craters, ranges of hills, and other small rilles.
It is unclear how exactly a lava flow would have followed such a path, and a variety of alternate theories have been presented for its formation. One popular theory is tectonic activity, making Rima Sirsalis a fault. However, it is not thought that there has been much tectonic activity on the Moon at the time when the feature would have to have formed. Another theory is that the groove is a collapsed dike of some sort. Dikes are igneous intrusions into weak rock, solidified lava that squeezed into a weak seam. This theory is not particularly well supported either.
Rima Sirsalis has another unusual feature, a strong localized magnetic field. The Moon's overall magnetic field is very weak in the present day, but it is thought that about 3.8 billion years ago the Moon may have had a global magnetic field as large as that of Earth. Rima Sirsalis may represent "fossil" magnetism from that era.