  We construct a compact, convex ancient solution of mean curvature flow in
$\mathbb R^{n+1}$ with $O(1)\times O(n)$ symmetry that lies in a slab of width
$\pi$. We provide detailed asymptotics for this solution and show that, up to
rigid motions, it is the only compact, convex, $O(n)$-invariant ancient
solution that lies in a slab of width $\pi$ and in no smaller slab.
