A magnitude 5.6 earthquake in the South Atlantic Ocean on September 29, 2026, established it as the day's largest event according to official monitoring data, driving the overwhelming 99.1% market-implied odds for the 5.5–5.6 bin. USGS and global seismic networks recorded no higher-magnitude quakes that day, with smaller events in California and elsewhere falling well below the threshold. Historical patterns show isolated mid-5 magnitude events occur regularly without precursors for larger ones, supporting trader consensus on this narrow outcome. A post-event magnitude revision upward or an undetected larger quake in a remote region could theoretically shift resolution, though dense global coverage makes such adjustments improbable.