International Ocean Discovery Program (IODP) Expedition 385T will revisit two Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legacy sites—Holes 504B and 896A on the Costa Rica Rift flank—to advance lithostratigraphic, hydrogeological, and deep biosphere studies of upper oceanic crust. Hole 504B has served as a standard reference site for upper oceanic crust for decades despite low core recovery during drilling operations. Hole 896A serves as an analog site of crustal alteration for examining biogeography in the crustal deep biosphere. During Expedition 385T, we will advance lithostratigraphic records of in situ crustal architecture through Formation MicroScanner (FMS) logging, with priority for these operations in Hole 504B. The new logs from Hole 504B will reveal whether unrecovered intervals are highly fractured and/or brecciated and whether alteration style and intensity are correlated to volcanic architecture, which will allow for assessment of the hypothesis that hydrothermal alteration and mineralization style are spreading-rate dependent. We will also advance crustal hydrogeological and deep biosphere research through temperature logging and water sampling in both holes, with priority for these operations in Hole 896A. The new FMS-based lithostratigraphy coupled with new fluid assessment will also allow for improvements on the thermal limits of microbial life and seawater-basalt reactions. These operations in Holes 504B and 896A have direct relevance to Challenges 5, 6, 9, 10, 13, and 14 of the IODP 2013–2023 Science Plan. To achieve these data and sample recoveries from these legacy sites, existing wireline observatories installed in both holes will be removed and the remaining cased holes will be left open for possible future installation of next-generation observatories. The expedition will be implemented as an abbreviated (10 operational days) expedition with no new coring.