The middle of the meninges. The most complicated. It has a bunch of cavelike open space that is full of CSF. There are ribbony things connecting across the open space holding the brain in place called trabeculae.

Cerebrospinal fluid (CSF) flows under the arachnoid in the subarachnoid space. 

Related to the ventricular system, made of four ventricles, which is where CSF is made. There are four  foramina connecting the ventricles, also connecting them to subarachnoid space via the cisterna magna and the cistern of great cerebral vein.