This paper provides a review of 1989-2003 designs and operations of the 20 Circulation Obviation Retrofit Kit (CORK) long-term subseafloor hydrogeological observatories installed in 18 holes during the Ocean Drilling Program (ODP). The basic configurations of the four models of CORKs developed during the ODP period are summarized: the original single-seal CORK (14 installations in 12 holes, 1991-2001) and three multilevel models, including the Advanced CORK or ACORK (2 installations, 2001), a wireline instrumented multipacker system or wireline CORK (2 installations, 2001), and the CORK-II (2 installations, 2002). The evolution of the scientific instrumentation installed in ODP CORKs and the history of postinstallation submersible operations are described. This instrumentation was provided by scientists with support of national ODP research funding, which also supported the extensive submersible time devoted to postinstallation data downloads and instrument servicing. Although the purpose of this paper does not include a review of CORK scientific results, we offer some comments on scientific lessons learned during the ODP CORK effort. We describe the funding and engineering support structure that held for the ODP CORK installations and close with some comments on the importance of engineering support for the Integrated Ocean Drilling Program goals involving long-term borehole observatories. We also provide a complete bibliography of CORK-related literature through 2004 and all of the data sets in digital form collected through 2003 from the six ODP CORK installations installed in either 1991 or 1996 near the Juan de Fuca Ridge, of which all but one are still in service.