The databases are sqlite. You can look inside them with many programs. I like to use:

SQLiteStudio from http://sqlitestudio.pl/.
The SQLite command-line shell program from https://www.sqlite.org/download.html.

I include the command-line shell with my normal frozen releases. I used to bundle SQLiteStudio as well, but they rewrote their gui and the program got too large to tuck in.

Feel free to explore how hydrus works, and if you have any questions or comments, don't hesitate to contact me!