\documentclass[margin=5mm]{standalone}
\usepackage{tikz}
\usepackage{xcolor}
\usetikzlibrary{patterns}

\begin{document}
\begin{tikzpicture}

\draw[fill=gray!33] [rounded corners=5mm]
(7,1) -- (5,1.5) -- (4,3) -- (1,2) -- 
(0,0) -- (1,-2) -- (7,-1) --
cycle;

\draw[fill=white] [rounded corners=3mm]
(4,2.5) -- (2,1.9) -- (3,2) -- cycle;

\draw[fill=gray!66] [rounded corners=4mm]
(6,1) -- (4,1.2)--(1,2) -- (0,0) -- (1,-1) -- (5,0) --
cycle;

\draw[fill=gray!33] [rounded corners=3mm]
(3.5,0) -- (4,1) -- (5,1) -- cycle;
cycle;

\draw[fill=gray] (2,1) circle (5mm);
\draw (2,1) node[]{$\bullet$};
\draw (2,1) node[below]{$a$};
\draw (1.5,1.5) node[]{$U$};
\draw (1.2,-0.5) node[]{$B$};
\draw (2,-1.2) node[]{$A$};
\end{tikzpicture}

\end{document}