def bottom(self, z: float = 0.0) -> Location:
        """
        :param z: the z distance in mm
        :return: a Point corresponding to the absolute position of the
                 bottom-center of the well (with the front-left corner of
                 slot 1 as (0,0,0)). If z is specified, returns a point
                 offset by z mm from bottom-center
        """
        top = self.top()
        bottom_z = top.point.z - self._depth + z
        return Location(Point(x=top.point.x, y=top.point.y, z=bottom_z), self)