How can I write multiple lines in a subscript?

You can use the \substack command, from the amsmath package, just like \text.

\[
  \underbrace{...}_{\substack{\text{Some long text that} \\ \text{should be multiline}}}
\]

• \substack, mentioned by Carsten, probably fits best to your \underbrace because of the centered alignment.

• subarray is similar but offers customizable alignment:

Example:

\[
 \underbrace{....}_{\begin{subarray}{l}\text{Some  long text that}\\
    \text{should be multiline}\end{subarray}}
\]

• \parbox also works in math mode. You could use a font size command inside.

Another possibility:

\[
 \underbrace{some equation}_{\text{Some long text that}\atop\text{should be multilined}}
\]