"Containment protocols are almost impervious to pre-Gravemind infestations. What could possibly go wrong?"
From Halo 3, "Isolation" is a map enclosed by large walls in an attempt to contain an infestation of the Flood. From the top, it is a lovely area; however, underneath, in sprawling caves, lie remnants of the vile beings. Likewise, St. Laurent Park was formerly a dump for the town of Somersworth. As is the case with many of these dumps, it has now turned into a big problem. (Note: The cache is located in an area unaffected by leakage.)
Fight the Flood with handy equipment such as the BR55HB SR, the Type-25 GL, the SRS99D-S2 AM, the always-helpful M90 CAWS, and the M274 ULATV. Watch out for Flood infestations! Good luck and good hunting!