For a history and background of Hueco Mundo, see the bleach wiki
Activities during the Messoris Threat
Hueco Mundo has recently been cultivating many groups of high spirit pressure individuals. Other than the usual hollow fighting hollow, these new groups have not disturbed the natural order of Hueco Mundo much. The Solari have slowly been creating a organized settlement reaching the size of a small town. The Messoris have constructed a flying castle that floats over the dunes of Hueco Mundo. The Anti-Messoris Militia has also started rebuilding Sosuke Aizen’s old fortress, Las Noches.