The "O" Theatre is built around a 1.5-million gallon pool that drops 17 feet below the rest of the stage. The theatre and the massive water tank were custom-built for Cirque during the construction of the Bellagio, before the casino's opening in 1998.

"O" features spectacular synchronised swimming, high dives, acrobatics and aerials, all of which takes place in, around and above the 1.5-million gallon pool.

Synchronised swimmers rehearse their routine in the afternoon, ahead of their nightly performance. There are two shows a night, five nights a week in the 1,800-seat theatre.

There are 14 safety divers in the pool for the duration of every performance of "O." Two of these divers wear full face masks so they can speak to the control booth during the performance.

During parts of the show, performers need to stay under water for as long as 90 seconds at a time. Regulators allow swimmers to breathe underwater during the performance, and 24 underwater speakers allow the SCUBA-certified performers to hear music and cues.

One of the "O" safety divers holds breathing regulators for the performers.

A giant umbrella, which serves as a boat in the "O" production, sits backstage.

The 53 by 90 foot stage is segmented into four parts, controlled by sophisticated hydraulic lifts. The seven lifts can raise the stage from the floor of the pool, 17 feet below, to 18 inches above water level.

The hydraulic controls backstage.

Stage level is just the beginning of the "O" performance. The show takes place from the depths of the 17-foot pool to 60 feet above the stage -- a team of four high-divers leaps from that height in every show.

High above the stage, four divers prepare to leap into the water below.

Up in the lighting booth, crew control every element of the show, with a view of what's going on backstage on the screens above them.

This massive rotating carousel in the scaffolding high above centre stage raises, lowers and rotates performers and props.

A technician works high above the stage, more than 60 feet up, preparing for the night's show.

Two hours before the show starts, the final props and set pieces are moved into place.

Performers wait for their cue beneath the water.

The final performance, featuring costumes by Dominique Lemieux.