The answer is "Volcanos".
The Sunda Strait isolates the islands of Sumatra and Java with the volcanic island Krakatoa situated in between between them.
The other Krakatau islets from the 1883 eruptions are known as Sertung, Panjang and Rakata. Java is a moderately little island contrasted with Sumatra, yet it has a higher convergence of volcanoes which are active.