Underbar nabla symbol doesn't work

\documentclass{article}

\begin{document}

$\underline{\nabla}$
\end{document}

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$\underbar{\hbox{$\nabla$}}$
\end{document}

or, alternately,

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$\setbox0=\hbox{$\nabla$}\underbar{\box0}$
\end{document}