Subseafloor borehole observatories (“CORKs”) are currently the best mechanism by which fluids from subsurface hydrologic zones can be collected to evaluate the composition, evolution, and consequence of fluid circulation in oceanic crust. The fluid-sampling capabilities of CORKs have evolved over two decades, spanning the Ocean Drilling Program and Integrated Ocean Drilling Program. The fluid-sampling system for the original CORK design consisted of a single polytetrafluoroethylene (PTFE) tube that connected to a valve at the seafloor and ended at depth in the formation. Through successes and disappointments coupled with community desires and efforts, significant iterations of CORK design and capabilities have led to the development of a range of crustal fluid-sampling systems. These iterations continue today with the development of new borehole capabilities, sensors, and samplers. This paper discusses these developments and transitions, highlighting the pros and cons of various designs, materials, and decisions. Although the evolution of CORK design has taken years because of the infrequency of CORK deployments and sample recovery operations, we as a community are now in a position to report on groundbreaking results that will enhance our understanding of subseafloor hydrogeology, crustal evolution, geochemical fluxes, microbial ecology, and biogeochemical processes, as indicated by the wealth of work referenced herein and by the complexity and flexibility of present and future designs.