Onr popular theory is Dr Walt Brown's Hydroplate Theory.
The basic premise of the theory is that most of the land was gathered together at one place. There were mountains, but perhaps 3,000 or 7,000 feet tall. At this altitude, there would be more than enough water to cover the entire Earth, considering the water both beneath the crust of the Earth and in the atmosphere.
When the flood occurred, and the "fountains of the deep" were opened, the mid-Atlantic ridge ripped open "like the seam of a baseball", and the water beneath the plate shot upward at supersonic speed. As the land eroded quickly at the seam, the mantle beneath began to bulge upwards, creating the mid-Atlantic ridge.
The plates, lubricated by the water beneath them, slid (the whole continent), reaching speads upwards of 45 miles per hour. When they reached near their current position, they encountered resistance, and the plates buckled, forcing them upwards in places such as the Rocky Mountains on America's West coast.
Dr. Walt Brown goes into much more detail on his site, with the full contents of his online book available to read or obtain a hard-copy.
The positives of his approach is that it systematically explains a very large number of natural phenomenea particular to our present Earth, including the fossil record, the different strata, the mid-Atlantic ridge, the mountain ranges, volcanos, asteroids and comets, and several high plateaus (including the Colorado Plateau), as well as many other features. He details his findings quite well, coming from a military career with degrees in Mechanical Engineering to back up his findings. He presents a strong case for a global flood.