使用 \includegraphics 时出现“缺少插入 endcsname”错误

Question

梅威瑟：

\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{float}
\usepackage{underscore}

\begin{document}
\section{Introduction} 

blurb goes here...........


\end{document}

\begin{document}这工作正常，但是当我在和之间添加以下图形代码时\end{document}，它就爆炸了：

\begin{figure}
    \includegraphics[scale=0.3]{flemish_bathy_50m.png}
\end{figure}

给出大量如下错误：

! Missing endcsname inserted.
<to be read again>
unhbox
l.15 ...graphics[scale=0.3]{flemish_bathy_50m.png}
The control sequence marked <to be read again> should
not appear between \csname and \endcsname.
! Undefined control sequence.
GenericError ...
#4 errhelp @err@ ...
l.15 ...graphics[scale=0.3]{flemish_bathy_50m.png}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Undefined control sequence.
GenericError ...
let @err@ ...
l.15 ...graphics[scale=0.3]{flemish_bathy_50m.png}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Undefined control sequence.
GenericError ...

png 文件与 tex 文件位于同一目录中。