It’s a mystery for the ages. Preliminary figures from immigration enforcement agencies indicate that the number of illegal border crossings in August was down. And not just down by a little, either. We’re talking about a roughly 60% decrease from the largest surge back in May. How on Earth did that happen? Politico has a few guesses.

President Donald Trump’s plan to force Mexico to stem the flow of migrants across the southwest border of the U.S. appears to be working.

Border arrests, a metric for illegal crossings, plummeted to 51,000 in August, according to preliminary government figures obtained by POLITICO Wednesday, down more than 60 percent since a peak in May. And border watchers say it’s largely because of an agreement Trump struck with Mexico in June. Mexican authorities, backed by the newly formed National Guard, are now cracking down on migrants traversing Mexico’s southern border with Guatemala, monitoring river crossings and stopping buses carrying migrants from Central America through Mexico. At the same time, the U.S. is making tens of thousands of asylum seekers wait in Mexico while their applications are considered.

Before we start breaking out the party balloons, let’s keep in mind that while this is a welcome improvement, 51,000 illegal border-crossing attempts is still far higher than should be acceptable. That works out to roughly 1,650 per day. Our immigration courts can’t process that many cases per day, so we’re still losing ground, though not as quickly.

Two other cautionary notes should be included here. First of all, we’ll have to wait and see if these lower levels are sustainable or if this was just a blip in the normal trends. Also, July and August are traditionally times when illegal crossings dip because of the high heat and harsh conditions in much of northern Mexico during the summer months. If we keep seeing figures like this into November and December we’ll have more reason to be confident.

But with all that said, this is still good news and demonstrates that the new policies arranged with the government of Mexico are having a positive effect. They’re working to cut off the flow of illegal migrants near their own southern border and keeping more migrants on their side of the border while they await court hearings. The word seems to be getting around in Central and South America that entering the United States illegally isn’t as simple as it used to be and it might not be worth the trip.

Still, the missing piece of the puzzle is the wall. A daunting physical barrier channeling people toward legal crossing points could still cut those numbers down massively. We don’t need to have the entire border walled off all at once. Some of the more inhospitable areas see very little traffic. But we could be doing far better than we are now. And we should make sure that happens.