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) The Schism appeared to consist of an opening into the Vortex, contained in a circular ring. Before the Schism lay a plaque bearing the Seal of Rassilon.

Ancient Gallifrey
When the Tenth Doctor's TARDIS brought him to ancient Gallifrey, billions of years before his time, he and Cindy Wu witness young Gallifreyans being forced into the schism by a group of Time Sentinels.

Some Gallifreyans believed that it was the direct exposure to the Time Vortex that caused Gallifrey to develop more quickly than other societies. (COMIC: Old Girl)

Use in Time Lord initiations
It was used by the Time Lords in an initiation ceremony for children entering the Time Lord Academy. At the age of eight, young Gallifreyans would be taken to the Schism. When subjected to its terrifying effects the children would react differently — "Some would be inspired, some would run away, and some would go mad." The Doctor and the Master agreed that the experience was painful. (TV: The End of Time) It was claimed prolonged exposure to the Schism over billions of years gave Time Lords their ability to regenerate and indeed was the primary factor in their evolution. (TV: A Good Man Goes to War)

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)

Initiation of the Master
As a child, during his initiation, the Master gazed into the Time Vortex through the Schism, and saw the raw power of time and space — "That's when the Master first saw eternity." (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.

Initiation of the Doctor
The Doctor suggested his eight-year-old self was one of the ones who ran away — indeed, that he never stopped running. (TV: The Sound of Drums) He also indicated that he found his experience painful. (TV: The End of Time)

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 ridiculously 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.