This Corrective Action Investigation Plan (CAIP) was developed for Corrective Action Unit (CAU) 99, Rainier Mesa/Shoshone Mountain. The CAIP is a requirement of the ''Federal Facility Agreement and Consent Order'' (FFACO) agreed to by the State of Nevada, the U.S. Department of Energy (DOE), and the U.S. Department of Defense (DoD) (FFACO, 1996). The FFACO addresses environmental restoration activities at U.S. Department of Energy, National Nuclear Security Administration Nevada Site Office (NNSA/NSO) facilities and sites including the underground testing area(s) of the Nevada Test Site (NTS). This CAIP describes the investigation activities currently planned for the Rainier Mesa/Shoshone Mountain CAU. These activities are consistent with the current Underground Test Area (UGTA) Project strategy described in Section 3.0 of Appendix VI, Revision No. 1 (December 7, 2000) of the FFACO (1996) and summarized in Section 2.1.2 of this plan. The Rainier Mesa/Shoshone Mountain CAU extends over several areas of the NTS (Figure 1-1) and includes former underground nuclear testing locations in Areas 12 and 16. The area referred to as ''Rainier Mesa'' includes the geographical area of Rainier Mesa proper and the contiguous Aqueduct Mesa. Figure 1-2 shows the locations of the tests (within tunnel complexes) conducted at Rainier Mesa. Shoshone …