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Hydrogen sulfide (H 2 S) is a toxic gas, and considerable research has been conducted for its control and removal from industrial wastewater and sewage water. However, no simple and practical technology is available for degrading H 2 S in situ at tunnel constructing sites. On May 11, 2020, an H 2 S blowout accident occurred in underground soil at a residential sewer-tunnel construction site in Iwakuni City, Yamaguchi Prefecture, Japan, filling the tunnel with high concentrations of H 2 S gas, causing the fatality of one worker owing to emphysema. River water flowing near the site was immediately introduced into the tunnel to trap the H 2 S gas, generating 652-m 3 water that contained high concentrations (120 mg/L) of dissolved H 2 S in the tunnel. To safely and quickly remove H 2 S in situ, the contaminated water was treated with high-density oxygen and ozone nanobubbles (O 2 /O 3 -HDNBs) generated using the ultrafine pore method. Consequently, H 2 S was removed from the contaminated water in 3 days. This is the first successful application of O 2 /O 3 -HDNB technology for the in situ oxidation of H 2 S in environmental water at a construction site. This study reports the practical application of this advanced technology and the system performance., (Copyright © 2021 Elsevier Ltd. All rights reserved.)