If Moses were around today and had to part the waters of Monterey Bay, he’d find no passage to the Promised Land. Instead, he’d find another Grand Canyon.

Below the waters of the bay lies Monterey Canyon, a chasm over a mile and a half deep. It starts narrowly off Moss Landing, widening and deepening as it spreads west into the open Pacific.

The food-rich waters have lured rare ocean dwellers like blue whales and other ocean-going creatures to within a few hundred yards of shore, attracting scientists from all over the world. But scientists find the immense forces that created the spectacular submarine canyon as mysterious as the deep, dark waters that fill the abyss.

Scientists offer three different theories to explain how the huge canyon was formed. One theory is that tens of thousands of years ago the Sacramento River emptied into Monterey Bay and it’s powerful outflow of sand and gravel carved the canyon. Another is that the canyon is a trace of an ancient earthquake fault that’s still buried below it. A third theory blames fresh ground water, from under the land, for seeping out of the sea floor and playing some role in forming the canyon.

"One of the old ideas was that all submarine canyons were formed from the rising and falling of sea level," said Gary Greene, a marine geologist with the U.S. Geological Survey in Menlo Park. He has studied the geology of the Monterey Bay area for about 25 years.

During the ice ages a lot of water was trapped as ice near the north and south poles. That hoarding of water by the poles lowered sea level and exposed the wide shallow continental shelf of the Monterey to the air. Some geologists have speculated that the Sacramento River once flowed through the Pajaro River gap (near River Oaks along Highway 129), emptied into the bay, carving a valley into the shelf. That valley could have grown during following ice ages, or it could have been enlarged underwater by grinding of mud and gravel that cascaded down the steep underwater slopes at the edge of the shelf. The problem is that although the Sacramento could have done the job, there’s little evidence that it did.

Greene favors another theory. He thinks the canyon may be the remnant of a very old earthquake fault that runs the length of the canyon but is now deeply buried. The ancient fault splits a layer of granite that is about 70 million years old. One side of the split was lifted higher than the other, forming a small valley. Over the last 10 million years that small fault valley -- and the block of earth it rides on -- was pushed up and out of the ocean and then submerged again several times.

When the fault valley was exposed to the air, a stream flowed through it and scoured it deeper. When the valley was submerged, mud, sand and gravel filled it up, but never all the way. Each time the top of the valley was in the air, there was enough of he valley left for water to follow it’s course and dig it out again and channel erosive debris down into the deep.

Meanwhile, the surrounding areas were getting thicker and thicker with sediments. Monterey Canyon was a weakness in the sediments that kept eroding away. Eventually the valley’s profile became more and more exaggerated, until today it is submerged again and is one of the biggest canyons in the world.

But there may be something else at work too. Daniel Orange suspects that in addition to the fault, fresh water seeping out of the ground from aquifers under the land might be ungluing the slopes, causing the canyon walls to weaken and collapse. Orange is a geologist at the University of California, Santa Cruz, and also works with the Monterey Bay Aquarium Research Institute.

An old rumor may bolster this theory, if it can be confirmed. Legend has it that more than a hundred years ago so much fresh water seeped out of the ocean floor, sailing ships came near shore in Monterey Bay and filled their drinking water casks by dipping them in the ocean where the fresh water "boiled" to the surface (fresh water floats on top of salt water, when they don’t get mixed). Seepage with that kind of pressure behind it may have weakened undersea slopes from within, causing landslides and eventually forming a canyon. But Orange isn’t ready to say seepage plays a major role in Monterey Canyon.

Scientists can only solve the mystery by exploring and testing Monterey Canyon itself. Orange will be going into the canyon for a closer look later this year in Monterey Bay Research Institute’s submersible vehicle "Ventana."

There’s a lot of work to do, and more researchers are getting interested in submarine canyons, said Greene. "Canyons can tell quite a story," he said, "and the serious work is just beginning."