Project A06 will study properties of extremely neutron-rich nuclei and address their understanding based on nuclear forces. We focus on the role of three-nucleon and in particular three-neutron interactions in loosely bound and unbound systems of extreme isospin. Besides the investigation of the tetra-neutron system, experiments will unravel the structure of extremely neutron-rich nuclei at and beyond the dripline around oxygen. Of particular interest are the investigation of neutron-neutron correlations in the two-neutron emitter 26O and the ground-state structure of 29F, which will provide information also on the structure of the unbound 28O nucleus. The experiments will be performed at RIBF, RIKEN, Japan.