Starting on a large, macro level, this melody is all f minor with a minor 7th.
While some of the melodic harmonies do not always line up with this interpretation, most of the structural elements do.
If we peal back one layer, we will notice that the object is oscillating between fmm7 and cmm7. The F pedal, due to the persistent bass line, is heard under the cmm7.
The Bb in the first measure causes some problem with this interpretation. Why, when all the other five measures of the bass line would this Bb occur here. Well, if the Bb were replaced with an Ab (like the other measures), there is a distinct localized clash between F major and F minor. In the other 5 measures the Ab is heard as a pedal harmony, but in the first measure, it would have been heard directly related to the melodic harmony and caused an audible dissonance that just sounds wrong.
Notice how this underworld theme is so short. You might recall that Super Mario Bros. also had a rather short underworld theme. These themes are slow, with minimal harmonic movement. They are designed to drive you crazy, just like the underworld areas are designed to be a bit darker than your typical overworld area.