EPA announced Thursday that the comment period for the Spruce No. 1 mine would be extended to June 4.
The comment period was to end Tuesday, but the agency says a website used to accept comments will be unavailable from Saturday through Monday because of maintenance.
EPA has said it plans to veto a previously granted permit for the mountaintop removal operation because valley fills would bury 7 miles of streams and hurt water quality.
EPA also says the project would affect 2,278 acres of forest.
The project was permitted in 2007, but has been delayed by lawsuits.
The Spruce No. 1 mine is owned by Arch Coal and is located in Sharples in Logan County.