- "For over a hundred years, Braavos hid itself from the eyes of the world, who called it the Secret City. Using a dye derived from a local snail, our captains stained their sails purple to hide their stolen Valyrian ships. Our merchants carried false charts and lied when questioned about their home port. Eventually, one sealord, our elected ruler, decided enough time had passed, and initiated the unmasking of Braavos to the world and to Valyria."
- ―Tycho Nestoris
Braavos was founded by slaves of the Valyrian Freehold who found shelter in a lagoon behind a wall of pine-clad hills and sea stones, where the frequent fogs would help hide the refugees from the eyes of dragon riders passing overhead. They built a city in which they vowed that slavery would not be practised, having risked their lives in the name of freedom. This became the first law of Braavos, engraved in a stone arch spanning the Long Canal.
For over a hundred years after this, Braavos hid itself from the world, leading many to call it the "Secret City". The captains of the city would hide their Valyrian ships by using a dye derived from a local snail to stain the sails purple. Braavos' merchants carried false charts and lied when questioned about their home port. Eventually, the Sealord of Braavos, Uthero Zalyne, decided that enough time had passed for Braavos to reveal itself to the world. The Iron Bank of Braavos paid the dragonlords for the stolen ships, but refused to pay for the value of the slaves themselves.
To this day, the anniversary of the Unmasking is celebrated in Braavos with a ten-day festival of feasting and masked revelry. It culminates on the midnight of the tenth day when the Titan of Braavos roars and tens of thousands of celebrants remove their masks as one.
In the books
In the A Song of Ice and Fire novels, the Unmasking is alternately known as the Uncloaking of Uthero. The Iron Bank of Braavos paid the settlements to the grandchildren of the owners of ships that Braavos's founders stole (making it a point, however, to not pay for the value of the slaves themselves). Ships were then sent forth throughout the world announcing the existence and location of Braavos and inviting men of all nations to the city to celebrate its 111th anniversary. The Valyrians accepted the money, but ultimately were more consumed with their own internal feuds and weren't really interested in the descendants of long-dead slaves living in an isolated city-state on the far borders of their territories, so they simply ignored Braavos and left it alone.
The Unmasking of Uthero is apparently inspired by the real-life famous Carnival celebration held annually in Venice since the 1100s. Like other Carnival/Mardi Gras celebrations worldwide it takes place the day before the Christian fasting period of Lent. Participants in Venice's Carnival celebrations famously wear masks throughout the festivities (so they can party and commit a range of vices free of inhibitions). Braavos, as a merchant city-state famed for its canals, is heavily inspired by real-life Venice.