Bionicle Adventures, tome 3 : The Darkness Below

The Toa Metru are nearly powerless after their defeat of the Morbuzakh but they cannot rest. Instead they must venture underground to save the city of Metru Nui's Archives. The Archives contain the history of the city, the Matoran and even the Toa. However, a flood has allowed all kinds of creatures that are normally kept in stasis there to escape and now the Toa must battle them to survive.