On April 6, just a week before the Osaka Expo kicks off, methane gas was detected at the venue, specifically in the Green World construction sector located on the west side. This discovery was made around 4 p.m. while rehearsals were underway and visitors were present at the site.

Following the detection, the Japan Association for the 2025 World Exposition made the decision to restrict access to the area shortly after the gas was found. To mitigate the situation, they opened the lid of an underground electric facility pit for ventilation purposes. About an hour later, the gas concentration was confirmed to have dropped below safety thresholds, allowing visitors back into the area and the resumption of rehearsals.

The venue itself is located on reclaimed land, where methane is produced due to the decomposition of waste materials buried in the area. An incident last March, where sparks from toilet construction ignited methane and caused an explosion, has heightened awareness of the gas's potential dangers. In response, safety measures such as gas detectors, ventilation systems, and natural ventilation options through drilled holes in manholes have been implemented. Despite these precautions, elevated levels of methane gas have still been identified in the Green World zone, including the day of the latest detection.