  We derive the sub-subleading two-pion exchange contributions to the
three-nucleon force which appear at next-to-next-to-next-to-next-to-leading
order in chiral effective field theory. In order to determine the low-energy
constants, a complete analysis of pion-nucleon scattering at the
subleading-loop order in the heavy-baryon expansion is carried out utilizing
the power counting scheme employed in the derivation of the nuclear forces. We
discuss the convergence of the chiral expansion for this particular
three-nucleon force topology and give the values of the low-energy constants
which provide the most realistic description of the three-nucleon force when
the chiral expansion is truncated at next-to-next-to-leading order.
