Sumatra, known to Arab geographers and mariners of the medieval world as ‘Lamri’, is the largest island entirely within Indonesia. Stretching for over a thousand miles from north to south, its backbone is a chain of jungle-shrouded volcano-mountains called the Bukit Barisan, whose rainforests shelter vestigial populations of the Sumatran tiger, elephant, rhinos and orangutans. It is for a glimpse of these critically endangered creatures, along with the vivid turquoise waters of a vast freshwater crater lake, Danau Toba, that most visitors travel to Sumatra. Despite having borne the brunt of the Boxing Day Tsunami of 2004, the island’s dazzling coastline also holds considerable allure, with some of the best snorkeling and diving in the region.