Geological survey, gravity measurements, paleomagnetic and geochemical studies have been carried out in the area of the Strait of Sunda. The Krakatau complex lies at the intersection of two graben zones and a north-south active, shallow seismic belt, which coincides with a volcanic lineament about 200 km long. The activity of this lineament coincides with a fracture zone along this seismic belt and commenced with extrusion of alkali basaltic rocks at Sukadana and shifted southward through Rajabasa, Sebsi, Sebuku and Krakatau. A 3000 m deep well was drilled in the Strait of Sunda portion of the Lampung Banten contract area. The well penetrated an apparently continuous sedimentary sequence ranging in age from Quaternary to Upper Pliocene. This area of the Strait of Sunda must have formed a rapidly subsiding trough, in which a vast thickness of clastic sediments accumulated, extending from Lampung to the Krakatau fracture zone. The gravity anomalies were observed (i) in the area of north of Ujung Kulon, indicating the existence of a low gravity caldera, from which the Malingping and Banten tuffs were ejected 0.1 Ma ago, and (ii) at the area of Kotaagung, where a graben structure was observed and an ignimbrite eruption occurred at 1 Ma. From the chemical analyses of volcanic materials, we deduced that the ignimbrite magma was generated by remelting of crustal materials. In the volcanic rocks along the volcanic lineament in the Strait of Sunda, no zonal arrangement comparable to the volcanic rocks in Java is recognized, but the chemical composition of Krakatau has a wider range than normal island arc volcanoes. Paleomagnetic studies suggest that the island of Sumatra has been rotating clockwise relative to Java from at least 2.0 Ma to the present at a rate of 5–10°/Ma. As the difference in strike of Java and Sumatra exceeds 20°, the rotation of Sumatra, and therefore the opening of the Strait of Sunda might have started before 2 Ma. The Strait of Sunda appears to be situated at a submerged volcano-tectonic depression in southernmost Sumatra which produced a large quantity of acid pyroclastics in the Late Quaternary.