Some browsers (Firefox in particular, also Google Chrome) have difficulty displaying several of these images (although Internet Explorer works well).
It’s best to download the maps, either singly (Option 2) or the whole site (above top),
and then use an image viewer (much more flexible and much faster than a browser; Irfanview is a great choice).
Option 2: Click on ZIP links to download the image, delivered to your hard disk in the ZIP file format.
UnZIP with WinRAR, WinZip, UnZip, or any unzip utility (freeware for Windows: FreeZip, 7Zip + ports for Mac and Linux, IZArc, PeaZip + Linux).
You can then view the maps in a more flexible external image viewer (preferably at native resolution), and access them permanently.
Irfanview is a superb, freeware graphics file viewer for Windows (only, unfortunately — or with assistance from Wine).
In Irfanview, hit the letter “F” to toggle between native (Full) resolution and a view which fits the entire map on your screen.
Either way, these files may take time to download.
Comments, criticisms, and suggestions are welcome.