Untempered Schism

The Untempered Schism was a natural opening in the space/time continuum, "a gap in the fabric of reality from which can be seen the whole of the Vortex". (TV: The Sound of Drums)

History
The Schism was used by the Time Lords in an initiation ceremony for children entering the Time Lord Academy. At the age of eight, the children would be taken to the Schism. (TV: The Sound of Drums) The Doctor and the Master agreed that the experience was painful. (TV: The End of Time)

When the Tenth Doctor's TARDIS brought him to ancient Gallifrey, billions of years before his time, he and Cindy Wu witnessed young Gallifreyans being forced into the schism by a group of Time Sentinels. (COMIC: Old Girl) During one initiation in the Dark Times, a group of students were thrown into the Schism. Their physical forms were splintered by unprotected travel through the Time Vortex. (COMIC: The Wishing Well Witch)

The Doctor said that, when subjected to the terrifying effects of the Schism, the children would react differently — "Some would be inspired, some would run away, and some would go mad." He identified himself as one of the ones who ran away and never stopped running; (TV: The Sound of Drums) others who ran away included Susan Foreman, Romana I, K'anpo Rimpoche, Drax, Azmael, the Corsair, and Outsiders such as Nesbin, Ablif, and Presta. (PROSE: A Brief History of Time Lords)

During the Master's initiation, he saw the raw power of time and space. (TV: The Sound of Drums) It was at this moment that the Drumming, a signal sent back through time by the Time Lords in the final hours of the Last Great Time War, (TV: The End of Time) entered the Master's mind and plagued him throughout the rest of his lives. (TV: The Sound of Drums)

Among those who were inspired were Savar, Runcible, Spandrell, Engin, Hilred, Kelner, Rodan, Maxil, Damon, Talor, Jerricho, Thalia, Hedin, Zorac, Flavia, Darkel, the Woman, the Partisan, Andro, Fabian, Kenossium, and Androgar. Some, such as the Monk, the War Chief, Goth, the Rani, Morbius, Borusa, Salyavin, and the Visionary, went mad. (PROSE: A Brief History of Time Lords)

The Tenth Doctor described looking into the Schism as the first time any young Time Lord "saw eternity". (TV: The Sound of Drums) The book of A included the quote, "When I was young, I looked through eternity's eye, and eternity created me!" (PROSE: What Keeps Their Lines Alive)

The Doctor noted on more than one occasion that it was direct exposure to the Time Vortex through the Untempered Schism that caused Gallifreyans to develop more quickly than other societies. Through billions of years, it was what caused them to become Time Lords. (COMIC: Old Girl, TV: A Good Man Goes to War, et al.)

Behind the scenes

 * The "time tunnel" effect seen in the Untempered Schism, and reflected in the eyes of the young Master, is the same technique used in the opening title sequence of the Doctor Who TV series from series 1 to the 2009 specials and during certain episodes from 2005 onwards where the TARDIS is shown in flight through the vortex. It was also used at the opening of The Sound of Drums to represent the transit through the space-time continuum with Jack Harkness's vortex manipulator.
 * The Untempered Schism is similar to the caldera from The Book of the War, which is a place on the Homeworld that acts as a node point where all lines of historical influence converge and one can stare directly into the heart of time.