documentclass{standalone}
usepackage{tikz,pgfplots}
pgfplotsset{compat=newest}
begin{document}
    begin{tikzpicture}
    begin{axis}[view={0}{90}]
    addplot3[surf, domain=-2:2, samples=30,colormap/cool] {exp(-x^2-y^2)};
    end{axis}
    end{tikzpicture}

    begin{tikzpicture}
    begin{axis}[view={0}{90}]
    addplot3[surf, domain=-2:2, samples=30,colormap/cool,
        opacity=0.2,
        fill opacity=1,
    ] {exp(-x^2-y^2)};
    end{axis}
    end{tikzpicture}
end{document}