Things didn't begin to change until the new millennium. In 1999, a possibly crazy geologist named Leigh Price wrote a paper for the United States Geological Service claiming that there were 400 billion barrels of oil in the Williston Basin, an exponential jump from earlier estimates. Price's reasoning was simple: He didn't think the oil was cached in a series of oil reservoirs but was spread across the Williston Basin in the middle Bakken layer about 10,000 feet under the Earth's surface. This would explain why so many Williston wells started so promisingly before petering out.
The USGS thought Price was nuts and refused to endorse his paper. It was only after oil prospectors began going through his research after his death in 2000 that the whispers began to spread. Technology caught up with Price's theory, proved him right, and oil companies began to see opportunity. Thanks to computers and minicameras mounted on the end of drills that helped keep you drilling right in the sweet spot, vertical drilling had begun to complement horizontal drilling. You still started your well by going straight down, but now once you hit the middle Bakken, you could bend your pipe, lay it horizontally for miles, and suck up the oil more efficiently.
Here's an analogy. Think of horizontal drilling as a dog sticking his long tongue into a peanut butter jar, sucking up every last speck of peanut butter.
America's the dog.