Seattle is sinking millimeter by millimeter, and new research shows it’s more widespread — and riskier — than once suspected. Why it matters: Land subsidence is an invisible but growing threat to urban infrastructure — cracking roads, destabilizing buildings and making low-lying areas even more flood-prone . In Seattle, it combines with sea level rise, seismic instability and aging buildings and infrastructure to heighten long-term risk. Driving the news: In a peer-reviewed study published Thursday in Nature, researchers analyzed six years of satellite radar data in the 28 most populous U.S. cities. They found that 25 of the 28…