Fort Jefferson, Florida is a large Civil-War-era island fortress (the largest masonry structure in the Americas), and is part of the Dry Tortugas National Park. This photograph - framed by the walls of the fort - captures the fort's brick moat, with the reef and the Gulf of Mexico beyond. The colors and horizontal layering in this photograph make it somewhat reminiscent of a work of abstract art.
https://en.wikipedia.org/wiki/Fort_Jefferson,_Florida
(March 16, 2013. Nikon D5000, 1/250s @ f/5.6, 22mm, ISO 200.)