Changuu Island lies just off the coast of Zanzibar, about 25 minutes from Stone Town by a small-engined boat. Though it only measures about 800 meters long and 230 meters wide, this narrow spit of land holds a lot of history.

The tiny island was uninhabited until the mid-1860s, when Majid bin Salid, the first Sultan of Zanzibar, sold it to several Arab slave traders using it to hold enslaved people in transit to Stone Town or beyond. After Zanzibar and Tanzania were deemed as British protectorates in the late 19th century, the island was bought by the colonial governor with the intention of building an isolated prison, giving it the nickname “Prison Island.” After the institution’s construction, however, its facilities were only used for quarantining visitors going between Zanzibar and the mainland, mostly holding people suspected of yellow fever.

Today, the island is owned by the government and acts primarily as an endangered giant tortoise sanctuary. After a 1919 gift of four tortoises to Changuu by Seychelles reproduced to more than 200 by the 1950s, the famed reptiles quickly outnumbered the human population of the island. These giant Aldabra tortoises could once be found across the Indian Ocean, but are now relegated to fewer and fewer conservation zones. The open-air sanctuary allows for supervised breeding of the tortoises and continued monitoring of the population.

These massive creatures are thought to be among the longest-lived animals on earth. (But it is difficult to verify the ages of many tortoises who have outlived the scientists studying them.) Currently, the oldest tortoise on Changuu Island is estimated to be 196 years old, predating the colonial occupation of Zanzibar and Tanzania by 65 years.