I THINK I have an approximate idea of what the problem is, though I'm not yet sure how to fix it. If you look at http://www.geosymbols.org/animals/reptiles/turtles/sea/ and scroll towards the bottom, you'll see that the footer is floating well above the bottom. (I want it resting on the bottom, like an obedient footer should.)
If you click "Related Links" (above the footer), the footer becomes properly positioned. So why does it float above the ground when "Related Links" is closed?
However, each set of links also has an ID matched to a particular page's ID number, which is also set in the style sheet (display: block). Thus, the following links would be visible on the Sea Turtles page only:
<div class="divinvis" id="divsea">
One of the reasons I think my linking system is the source of the problem is the fact that all the pages in the Turtles section appear to be OK except for Sea Turtles and Box Turtles, which have the most links.
I suspect the solution to my problem may be obvious. If it is, please share it with me!