The Crossing by Bill Viola consists of a large screen which displays a motion picture on both sides (front and back) simultaniously. It is hung in the middle of a room so viewers can walk around it if they wished. On one side, the video shows a man walking towards the audience in slow motion. He comes to a halt, and moments later a fire starts at his feet. The fire climbs up his body until the man is entirely covered in flames. When the fire slows again, the man is gone, guzzled by the fire.
On the other side of the screen, the same man walks towards the audience in slow motion. But when he stops this time, water starts dripping on his head. Gradually, more and more water falls down on him until finally he is not visible anymore, completely hidden behind the water. Again, when the water stops falling, the man has disappeared. All this was accompanied by matching sounds.
Video documentation of The Crossing and making of the work can be found on the SF MOMA website: http://www.sfmoma.org/multimedia/videos/11
Viola's personal history unveils why his art (almost) always has something to do with water. When he was six years old, he fell out of a boat when he was on holiday with his parents. He nearly drowned. But unlike most people, Viola described the experience as “… the most beautiful world I’ve ever seen in my life” and “without fear,” and “peaceful” . This near-drowning experience resulted in his fascination with water.
"Often I've used water as a metaphor, the surface both reflecting the outer world and acting as a barrier to the other world." 
For The Crossing, Viola added another element: fire; water's natural opposite. This results in an interesting interplay between antipoles. Tung Nguyen says this piece is about balance. "The concept of balance is presented through opposite concepts that each alone is an extreme that can only be balanced by one of the other extreme. Left - right, fire - water, red - blue, warm - cool, hot - cold, bottom - top, and so on. The viewer can even sense the artist's desire for moderation, for compromise, and the message that the extremes are what harming us (burnt by fire or washed away by water) and we need a common ground to survive." 
But one might also think about the disappearance as the most important part. Where has the man gone, is he dead? Or is he in a higher state? Perhaps he has dissolved on to the other side of the screen: first catching fire to be put out by the water later or first being soaking wet and in need of some warmth. In that case it is more a "what-came-first-the-chicken-or-the-egg" kind of question. Who is to say? One thing is certain: Bill Viola says the idea came out of his unconscious. Since we can't look there for answers, we should probably interpret this piece on our own, with help of our own unconscious part of our brain.
In the following video, the artist explains how he got the idea and why he just had to make this piece:
In Viola's essay, 'Will There Be Condominiums in Data Space?' (1982) , he brings up the subject of the 'brain'. He says: "In our brain, constantly flickering pulses of neuron firings create a steady-state field onto which disturbances and perturbations are registered as percepts and thought forms. This is the notion that something is already 'on' before you approach it, like the universe, or like a video camera which always needs to be 'video-ing' [...]." This explains how the idea got into his head: it was at first pulses of neuron firings which turned into thought forms. The brain is always on, you can't switch it off. The idea was just pulsed into his head. So basically, when we are watching this artwork, we are looking at an adaptation of random neuron firings in Bill Viola's head.
 Bill Viola: The Eye of the Heart. Dir. Mark Kidal. DVD. Film for the Humanities & Sciences, 2005. Quote on Wikipedia
 Shanken, Edward. Art and Electronic Media (Phaidon 2009): p. 219