Today, we spend a few minutes reviewing the recent history of housing markets in rural North Dakota. In a microcosm, we can see how the interplay of demand and supply drive housing market cycles. The speed and scale of changes in North Dakota dwarf what we usually see, but provide an illustration of the forces at work in many cities around the country.

For most of the past decade, the real estate market in Williston, North Dakota has been on an amazing tear. The region, home to the Bakken shale formation, has been the epicenter of the U.S. oil fracking industry. With sustained oil prices in the $100 a barrel range, everyone from global energy companies to independent producers have been drilling exploratory and production wells, and state’s oil output increased by a factor of ten, from about 100,000 barrels per day to more than 1,000,000 barrels per day.

Job growth quickly overwhelmed the local housing supply, spiking rents, and leading landlords to rent out travel trailers, garages, storage units, and outbuildings to oil workers—and those in the local service industry that grew in response to the population influx. Williston even became famous for “man camps”—quickly assembled fields of trailers and modular housing units, inhabited almost entirely by male oil workers.

But in the past year, everything has changed. First, the oil market has gone bust, with prices falling from more than $100 to recent lows of less than $38. In response, oil companies have drastically cut back on exploration and new well-drilling. The industry is shedding jobs.

Second—and importantly for our story—the local market has seen an incredible surge of new housing construction. The number of building permits issued in Williston grew ten-fold between 2009 and 2013.

The combination of flagging demand and a newly abundant supply has rental prices in Williston dropping like a rock. According to real estate analytics firm Zillow, average rents in the area have declined by 23.4 percent over the twelve month period ending in January 2016. Reuters reports that new apartments which were commanding rents of as much as $3,200 per month have now discounted rents sharply, added communal hot tubs and are providing free alcohol and snacks for residents.

The Williston experience provides a dramatic, but very clear, example of the dynamics of local real estate markets. The critical issue here is what you might call a “temporal mismatch” between demand and supply. Demand is the hare; supply is the tortoise. Demand can change in an instant—as quickly as new jobs open up, and as quickly as U-Hauls and moving vans deliver new residents to a city (or neighborhood). Supply takes time: planning, gaining financial and zoning approvals for new units, and then actually building and finishing out apartments and houses takes as much as 18 to 24 months. And when demand continues to change, supply can be struggling to keep up.

That’s just what happened in Williston. Developers did respond early on (building permits more than quadrupled in 2009 and then doubled in 2010) but demand grew even more, with the result that there continued to be a rise in rents.

Now, finally, due to the combination of flagging demand and the relentless (if comparatively plodding) increase in supply, the market is much closer to balance.

Williston is, in a supercharged microcosm, a metaphor for the housing market in US cities. In the past decade, the demand for rental housiing and urban locations has far outstripped the growth in supply. Lots more people have decided for economic reasons (or because they prefer city neighborhoods that they want to rent in cities. And housing supply, at least initially, has hardly budged—first because of the aftermath of the Great Recession, and still, in many cities because of local zoning restricts and slows the number of new housing units that can be built. But in true tortoise fashion, supply is beginning to catch up to demand. As we’ve seen in Denver, Seattle, and Washington, when a sufficient number of new apartments are built, they begin to shave rent inflation. The Williston story is also playing out in another oil-town: Houston, where new apartments are going begging and landlords are offering free first month’s rent to new tenants.

We think there are two key takeaways here. First, supply and demand do operate: building more housing is the key to addressing rental affordability. Second, housing markets are inevitably subject to a temporal mismatch between supply and demand. Unlike the neat whiteboard drawings of supply and demand curves that you may have seen in an undergraduate economics lecture, which can be erased and redrawn in a moment, in the real estate market, demand is fast, but new supply is slow.