When I did a PhD in theoretical/numerical physics, I wrote most of my code in Fortran. That was started ~5 years ago.
Basically, I was starting to port some some Matlab code I had written to C/C++ to speed it up, but the most convenient solver for the differential equations I was dealing with outside of Matlab turned out to be a Fortran library. So I started learning basic Fortran in order to write a small C++ wrapper around that library, but ended up liking Fortran much better than C++, so I kept using it throughout my PhD.
Note that modern Fortran (2008+) is not at all like the infamous Fortran 77. There's no reason to write GOTO spaghetti; you have object-orientation, built-in support for complex numbers and matrices, array slicing, pointers, polymorphism, pure functions, vectorized ("elementary") functions and subroutines, and other modern features. There are even some unique features like the "associate" keyword that I sorely miss when doing mathematical programming in any other language (mostly Python these days). Because e.g. the built-in arrays "know how big they are", error messages are typically more helpful than debugging numerical C++ too.
Basically, I was starting to port some some Matlab code I had written to C/C++ to speed it up, but the most convenient solver for the differential equations I was dealing with outside of Matlab turned out to be a Fortran library. So I started learning basic Fortran in order to write a small C++ wrapper around that library, but ended up liking Fortran much better than C++, so I kept using it throughout my PhD.
Note that modern Fortran (2008+) is not at all like the infamous Fortran 77. There's no reason to write GOTO spaghetti; you have object-orientation, built-in support for complex numbers and matrices, array slicing, pointers, polymorphism, pure functions, vectorized ("elementary") functions and subroutines, and other modern features. There are even some unique features like the "associate" keyword that I sorely miss when doing mathematical programming in any other language (mostly Python these days). Because e.g. the built-in arrays "know how big they are", error messages are typically more helpful than debugging numerical C++ too.