Bellhop: what is a .bty file and how do I make one?

I made one using a env. but it won't work and I honestly don't know where you would add a .bty extension. Here is the error message I got when trying to use a env. file:

Using bottom-bathymetry file

BTYFile = finfish1.1.bty

*** FATAL ERROR ***

Generated by program or subroutine: ReadBTY

Unable to open bathymetry file