From The Stargate Omnipedia
Ancient stasis unit
Device of Ancient
design, capable of freezing a human body so that it can be maintained indefinitely. One of the units was discovered in the Atlantus outpost
in Antarctica -- the former home of Atlantis
, before it left Earth
and traveled to the Pegasus Galaxy
Atlantis itself also has a number of stasis units, which have been utilized by the expedition from Earth under emergency circumstances.
The Ancient Ayiana may have been placed inside this type of device to prevent her death when her people left her behind on Earth between five and 10 million years ago, when the city of Atlantis left the planet. She was later recovered by S.G.C. personnel and revived.
Lost City, Part 2 - Jack O'Neill places himself inside the Ancient stasis unit to sleep until something can be done to save his life.
Before I Sleep - Thrown back in time, Dr. Weir is put in stasis in Atlantis for 10,000 years, and is discovered by the Atlantis expedition.
The Kindred, Part 2 - His body rapidly deteriorating, a clone of Dr. Carson Beckett is put into stasis in Atlantis until a cure for his condition can be found.
The Last Man - John Sheppard is thrown tens of thousands of years into Atlantis's future, and uses the stasis unit to wait out the next 700 years until a solar flare can send him home.
The Seed - Carson is brought out of stasis when Dr. Keller finds a cure for his condition.