Underbar nabla symbol doesn't work

Question

I want to write a symbol like the one shown in the following image

but I can't make it work. I tried:

$$ \underbar{\nabla} $$

but it didn't work (I think the reason for this is that the input for the underbar command should be only text.

How can I make this work ?

Answer 1

\documentclass{article}

\begin{document}

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

Answer 2

\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}