I actually have a surprising amount of experience in Fortran, as both of my major "research" projects in college had to do with legacy Fortran code bases. The first was fitting a series of potential energy calculation libraries with Mathematica bindings, and the second was converting a library that did a particular kind of blur and glob detection to FITS datasets to C.
The second one was extra fun because the library was actually written in "FORCE", an extensive macro-extension of Fortran, for which we no longer had a compiler. All I had was docs, the original published paper, the original source, and a handful of input-output examples on substantial datasets
(for context, the datasets were essentially a 1024 x 1024 x 1024 array of floats that needed to be convolved with two different Gaussian kernels and compared pixel by pixel in an interesting way).
The second one was extra fun because the library was actually written in "FORCE", an extensive macro-extension of Fortran, for which we no longer had a compiler. All I had was docs, the original published paper, the original source, and a handful of input-output examples on substantial datasets
(for context, the datasets were essentially a 1024 x 1024 x 1024 array of floats that needed to be convolved with two different Gaussian kernels and compared pixel by pixel in an interesting way).