The chiseled and scorched area in the center of the image is where a five line ancient Hebrew inscription was found in 1880. The inscription is now in a museum in Istanbul.
The inscription describes how two groups of men began working at each end of the tunnel and how, after a bit of difficulty, they met in the middle. At that time the water began to flow from the Gihon Spring to the Pool of Siloam and has ever since!
This space is about 18 in. [45 cm.] high and 2 ft. [60 cm.] long.
For a translation of this inscription see Arnold, Bill T., and Beyer, Bryan E. eds. Readings from the Ancient Near East: Primary Sources for Old Testament Study. Grand Rapids, MI: Baker Books, 2001. To view for possible purchase Click Here.