x-underline-at-descent-line

x-underline-at-descent-line is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means to draw the underline at the same place as the descent line.
A value of nil means to draw the underline according to the value of the
variable `x-use-underline-position-properties', which is usually at the
baseline level. The default value is nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.