The basic reason for extreme hardness is the three-dimensional arrangement of carbon in corner sharing C4 tetrahedra with d(C-C) = 1.55 Å, i.e. the sum of two r(C) = 0.77 Å atomic radii, making a perfectly covalent network. In recent decades many research efforts were focused on the identification of new ultrahard carbon allotropes mimicking structure through predicting software such as CALYPSO and USPEX complementarily with first principles calculations carried out mainly within the well-established quantum mechanics framework of the density functional theory (DFT) . In this context the SACADA database (SACADA 3D) regroups all known carbon allotropes thus helping researchers in their endeavor. Recently we reported body-centered tetragonal C 4 (Fig. ) proposed in space group I-4m2 as seed to build larger carbon networks and to serve as a template for other original chemical compounds . Note that by considering the primitive cell (2 atoms), tet-C 4 is another expression of cubic diamond whose presentation as tet-C 4 is shown here as a starting building block to propose a novel carbon allotrope. In Table showing the crystal structure parameters of tet-C 4 , the change of two-fold carbon (2b) by four-fold position leading to the separation of the C4 tetrahedra along vertical direction keeps the same body-centered tetragonal space group I-4m2 after full geometry relaxation onto ground state structure leading to tetragonal C 6 (Fig. ). The transformation scheme highlighting the C4 tetrahedra is proposed in Fig. . Tetragonal C 6 is characterized by mixed C(sp 2 )/C(sp 3 ) hybridizations and reveals exceptional mechanical properties (very high hardness and elastic moduli) while being dynamically stable with electronic conductivity due to trigonal C(sp 2 ) atoms.