The results of drilling the Toa Baja hole will help answer some fundamental questions about the evolution of the Puerto Rico area. What is the thermal history of the arc massif, and its relation to deep and shallow fluid circulation, and the cooling effects of the down-going slab? What were the sedimentary environments of Eocene, Oligocene and Miocene time, and what is the origin of the North Coast Tertiary Basin? What is the nature of organic matter and fluids in the borehole, and their relationship to paleoenvironments of deposition, thermal regime, and flow through porous media? What are the stress orientations and fracture characteristics in the North Coast Tertiary Basin, and their relation to regional tectonic motions?