Ensure you have downloaded every part of the set (SeaMaps.7z.001, SeaMaps.7z.002, etc.).

Multi-part 7z files work like a puzzle; you cannot open just one piece (like .003) on its own.

Place every part in the same directory on your computer. Use 7-Zip: Download and install 7-Zip if you don't have it. Right-click only the first file (the one ending in .001 ). Select 7-Zip > Extract Here or Extract to "SeaMaps" .