Generate an image depicting Poseidon, the mighty God of the Sea, riding a magnificent horse beneath the depths of the ocean. The scene should be enveloped in a mystical atmosphere, shrouded in mist and swirling currents. The image should capture the sense of adventure and mystery that accompanies Poseidon's presence.

Poseidon, with his muscular build and flowing beard, is astride a majestic white horse, whose mane and tail billow gracefully in the water. The horse possesses a mythical quality, its eyes glowing with a captivating luminescence that pierces through the surrounding darkness.

As Poseidon and his ethereal steed traverse the underwater realm, the water around them takes on an otherworldly glow, casting an eerie light on their path. The misty atmosphere adds an air of intrigue, veiling the surrounding landscape in a soft, enigmatic haze.

The underwater terrain is a blend of ancient ruins and vibrant coral reefs. Remnants of forgotten civilizations rise from the ocean floor,