Photos taken in September 1957 at the military base Fort Belvoir in Virginia show a ring type UFO which produces a vapor which eventually engulfs it.

There has been some debunking of the photos over the decades even by the military itself, but honestly, I think its a genuine aircraft of unknown and likely extraterrestrial origin.

A report on the case with photos.

http://www.angelfire.com/wa/UFORC/page63.html

Rense.com has scans of a magazine article on the incident.

http://www.rense.com/general3/ring.htm

There is a lot of these at airshows or places with a lot of plane activity.
Do you know why? They are rings of smoke left created by explosions.

I read a big article about these in UFO Magazine, before it closed.
They do not stay as you have shown above in a perfect ring, they move around and distort like a flexible rubber band.

They stay together because of the chemicals in them.

There is another possibility.

The ring is a conducting coil, like a stargate through which another ring would resonate with, creating a worm hole through space and time.