The lower core was a section of Deep Space 9 and other Nor-class space stations. The lower core was situated at the bottom of central core section and at it's widest point 184.46m in diameter. The lower was effectively the engineering section of the station, the bottom section of it being taken up with the fusion core the station's power plant, which housed six fusion reactors and could be detached in emergency situations. (DS9 reference: Technical Manual; DS9 novels: Avatar, Book One, Avatar, Book Two)
In the initial assembly of Terok Nor the lower core was the first section to begin construction following assembly of mid core, the lower core's fusion core was then added once the upper core had begun construction. Like the rest of the central core sections the lower core's structure was composed of 31% kelindide, 65% rodinium and 4% toranium dicorferite, all of which was sourced from Bajor. (DS9 reference: Technical Manual)