Known affectionately as 'the watch with a hole', the Aquis Depth Gauge uses the scientific principles of the Boyle-Mariotte Law to create a gauge that clearly measures depth during a dive. The patented system works by allowing water through a hole cut into the sapphire crystal at 12 o'clock and into a channel milled around the outer edge of the crystal. This creates a watermark that corresponds to a gauge clearly indicated in yellow on the dial.