There are lot of ways to try extracting a dark feature in ENVI. The best one will depend on how you can distinguish the feature (e.g., by spectrum, by shape, by value in a specific band, etc.) from its background. Also on exactly what you want to end up with (e.g., an image with all pixels of that feature assigned a specific color? Or maybe a vector polygon for the feature?). You might want to look into ENVI's Classification Workflow tool. Or ENVI's Feature Extraction tools. - Peg Exelis VIS