\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
\begin{axis}[domain=-10:10,view={0}{90}, samples=6] 
\addplot3[contour gnuplot={number=15,labels=false},thick]  {(y^2)/2-cos(deg(x))};
\end{axis}
\end{tikzpicture}
\end{document}
