This is an amazing inversion of the purpose of semacodes. It is placed in the real world, but in order to see it and decode it, you have to use Google Earth.

(description from: http://hello.w0r1d.net/)

“Hello, world!” is a real installation for the virtual globe of

the software Google Earth. A Semacode measuring 160 x 160 meters was mown into a wheat field near the town of Ilmenau in the Land Thuringia. The code consists of 18 x 18 bright and dark squares producing decoded the phrase “Hello, world!”.

The project was realized in May 2006 and photographs were taken of it during a picture flight in the following month.