Ultra-deep riser drilling is planned for the seismogenic zone deep down beneath the seabed, across the plate boundary where the oceaninc crust subducts at estimated depths of 6,000 to 7,000 m below the seabed, in order to elucidate the mechanism of great earthquakes. In this expedition, the base for the ultra-deep riser drilling borehole was opened and the wellhead was installed on the seabed.