Venice sits at the northern end of the Adriatic Sea. The Venetian lagoon acted as a defensive moat, making Venice nearly impervious to invasions from the sea, while the treacherous mud and shifting topography of the lagoon were fatal to the land armies of invaders. With a mean depth of 1.5 meters most of the lagoon is navigable only by small, flat-bottomed boats. Several channels up to 30 meters deep allowed large merchant galleys egress to the Adriatic and Mediterranean Seas, bringing the riches of Asia to the growing European markets for luxury goods such as spices, silks, and gems. (Venice was also a major slave trader until the Rialto market was closed to the slave trade in 1366.)