During his campaign for the presidency, one of Donald Trump’s signature promises was that he would build a “great great wall” on the border between the United States and Mexico, and Mexico was going to pay for it. A year and a half into his term, with only a few prototype segments erected, the wall is the 2,000-mile multibillion-dollar elephant in the room of contemporary American life.

In The Great Great Wall, architectural historian and critic Ian Volner takes a deep dive into the story of Trump’s wall. Volner follows the conception, selling, design, and construction (or lack thereof) of this expensive and consequential barrier, giving listeners a detailed description of what’s happening in Washington, DC—and along the border. He also travels far afield, to China, the Middle East, northern England, and back to our border to examine the barriers we’ve been building for centuries. Why do we build walls? What do they reveal about human history?