A matter-isolating system for use in combination with an underground storage tank assembly comprises primary and secondary spillage containment assemblies and a double-walled manhole assembly. The manhole assembly comprises a manhole lid, an outer skirt wall, and an inner skirt wall. The inner skirt wall defines an inner manhole cavity and an outer manhole channel. The outer manhole channel directs moisture from the lid to certain backfill material thus isolating the inner manhole cavity from channel-directed moisture. The spillage containment assemblies form concentric primary and secondary spillage-containing chambers in radial adjacency to a tank access conduit of the underground tank assembly. The inner manhole cavity functions to isolate the secondary spillage containment chamber from the outer manhole channel and the primary spillage containment chamber isolates the secondary spillage containment chamber from the tank access conduit. A chamber-monitoring system may be incorporated into the system for monitoring the isolated chambers.