Nearly one in four homes in Puerto Rico is vacant - a 23.1% vacancy rate in 2025, up from 19.4% in 2010 - while the United States as a whole moved the other way, falling from 13.1% to 9.6% over the same fifteen years. The gap between the island and the mainland widened from 6.3 percentage points to 13.5. Since 2018, the year after Hurricane Maria, Puerto Rico has ranked number one in vacant housing among all US states and territories, losing the top spot only once, in 2022, when Maine briefly edged past it. The scale alone is not the most telling finding. What separates Puerto Rico from other high-vacancy jurisdictions is what kind of vacancies they are. In Maine, 72.6% of vacant units are seasonal or recreational; in Vermont, 77.1%. Puerto Rico's seasonal share is just 21.7% - 81,611 of its 375,748 vacant units - below the 30.0% national average. These are not mostly vacation homes. Even more striking: only 14.3% of Puerto Rico's vacant units are actually linked to the housing market - listed for rent or sale, or recently sold or rented but not yet occupied. The national share is 35.0%, more than double. An island with 375,748 empty homes offers roughly one market-available vacancy per seven empty units, which speaks directly to the island's chronic housing-affordability crisis: the inventory exists, but it is not for sale and not for rent. The largest single category in Puerto Rico is the Census Bureau's catch-all "Other vacant": 240,344 units, 64.0% of all vacancies, versus 34.6% nationally. Analyst Marcos Valdez's review of the ACS data flags exactly this anomaly - only Kansas and Mississippi sit in the same national range, and Puerto Rico's share is more than double that of Alaska, the next-closest high-vacancy state. What "other vacant" means on an island losing population is the open question, and the one the next round of analysis has to answer. The analysis draws on American Community Survey 1-year data from 2010 through 2025 (tables B25002 and B25004), and its arithmetic is transparent throughout. Puerto Rico's 23.1% rate puts it at 19.5 points above West Virginia, the fifth-highest state - a margin that has grown in almost every year of the series.