The Oman ophiolite drilling project under the International Continental Scientific Drilling Program was carried out in 2016-2018. A team of Japanese scientists led efforts to drill through the oceanic crust–mantle boundary that formed the ocean floor more than 100 million years ago. They succeeded in collecting continuous core samples from the gabbro sequence (the lowest part of the oceanic crust) through the Moho transition zone to the harzburgite sequence (the uppermost part of the mantle), and obtained fresh core samples (left photo) from the crust–mantle boundary. Many of the shiny particles that can be seen with the naked eye are crystals that were originally olivine under the mid-oceanic ridge and have been serpentinized by secondary alteration. Right photo: The drilling rig shown at the center of the photo was brought to the site, and core samples with a maximum length of 400 m were collected from two drilling holes. Two rotary holes were also drilled to measure physical properties in the holes. The core samples collected include gabbro, which constituted the lowermost part of the oceanic crust, through the Moho transition zone, and harzburgite, which constituted the uppermost part of the mantle. This crust–mantle boundary drilling is an important milestone in our ongoing project to drill into the mantle at the bottom of the ocean.