To better understand the dynamics of the Earth's interior, we need more observational data from the planet's oceans. In particular, we lack time series data, which require long-term observations. It is essential to probe the Earth not only from the seafloor but also from sub-seafloor boreholes. The reasons are twofold. First, there are geodetic and seismic measurements that should be made in a low-noise environment, preferably within competent rocks. Second, there are many critical measurements that can be made in situ or close to where active bio-geological processes occur. In this regard, we are at a stage where we may make substantial progress. In recent years, a number of successful emplacements of borehole observatories have been made. In 2006, a new type of scientific drilling vessel equipped with a riser capability and blowout prevention to allow deeper drilling than available at present will become part of a major international drilling program. Borehole observatory constructions will be accelerated to complete a truly global seismic network for obtaining clearer tomographic images, to establish regional geodetic networks to monitor strain buildup at plate boundaries, or to set up local multiple-component observatories to understand active processes at hydrothermal vents, fluid flow pathways, or seismogenic zones.