<p>I've recently come to maintain a large amount of scientific calculation-intensive FORTRAN code.  I'm having difficulties getting a handle on all of the, say, nuances, of a forty year old language, despite google &amp; two introductory level books.  The code is rife with "performance enhancing improvements".  Does anyone have any guides or practical advice for <strong>de</strong>-optimizing FORTRAN into CS 101 levels?  Does anyone have knowledge of how FORTRAN code optimization operated? Are there any typical FORTRAN 'gotchas' that might not occur to a Java/C++/.NET raised developer taking over a FORTRAN 77/90 codebase?</p>

Well, in one sense, you're lucky, 'cause Fortran doesn't have much in the way of subtle flow-of-control constructs or inheritance or the like.  On the other, it's got some truly amazing gotchas, like the arithmetically-calculated branch-to-numeric-label stuff, the implicitly-typed variables which don't require declaration, the lack of true keywords.
I don't know about the "performance enhancing improvements".  I'd guess most of them are probably ineffective, as a couple of decades of compiler technology have made most hinting unnecessary.  Unfortunately, you'll probably have to leave things the way they are, unless you're planning to do a massive rewrite.
Anyway, the core scientific calculation code should be fairly readable.  Any programming language using infix arithmetic would be good preparation for reading Fortran's arithmetic and assignment code.