The crash in oil prices kicked off intense debate over when, and how, American producers would react. So far they’re still cranking out oil, but there are signs that a slowdown is looming. Chief among them: the record drop-off in drilling for new oil. The animation below shows the deployment of drilling rigs since 2011, culminating recently in a sudden collapse.

These four years of data represent the fastest expansion of oil production in U.S. history. New technology drove this boom—particularly the deployment of horizontal drilling through shale rock. The three biggest oil-producing shale regions are the Permian basin in West Texas, the Eagle Ford in Southern Texas, and the Bakken in North Dakota. Hover on the map for drilling history in each basin.

As the price of oil plunged in the second half of last year, producers started shutting down rigs at an unprecedented rate. First to go were rigs that operated without long-term contracts. Then companies began terminating agreements early and letting expired contracts go unrenewed. Active rigs declined by more than 40 percent. Remaining drilling efforts have focused on the most productive oil regions—sweet spots where oil will be profitable even at $40 a barrel.

So with all these drilling rigs shutting down, why isn’t oil production slowing? The short answer is efficiency. (See the longer answer here.) New wells pump more oil faster, so raw rig counts are losing some of their predictive power, at least for now. In fact, despite the tumbling number of active rigs, the U.S. is pumping more oil than any time since 1972.

Producers are running out of places to put all that oil. Storage-tank levels are at record highs, and production could drop sharply if inventory space ever maxes out.

Just as worrisome to oil investors is the so-called fracklog—thousands of nearly completed wells that are being left untapped. As soon as oil prices start to rise, these wells can start pumping crude after just a few weeks of finishing work. The fracklog has become America’s de facto backup storage.

Rig declines are likely to continue for the next few months, and may not stabilize until the third quarter, according to an analysis by Bloomberg Intelligence. In three previous oil slumps, it took about 32 weeks for rigs to bottom out. In highly productive shale regions with big fracklogs, production could still climb for months even if rig counts fall to zero.

For drilling companies, it’s an industry in crisis. Thousands of jobs have been lost, and more will follow. But for countries like the U.S. that buy more oil than they produce, the economic benefits of cheap oil and cheap gasoline outweigh the economic losses.