Matsya Purana — The Cave-Sanctuary: Jewel-Lake
मकराणां च मत्स्यानां चण्डानां कच्छपैः सह तत्र मरकतखण्डानि वज्राणां च सहस्रशः //
makarāṇāṃ ca matsyānāṃ caṇḍānāṃ kacchapaiḥ saha tatra marakatakhaṇḍāni vajrāṇāṃ ca sahasraśaḥ //
There are makaras, crocodile-like, and fierce fish together with tortoises; and there, in their thousands, are fragments of emerald and also vajras (diamonds).
This verse does not describe Pralaya directly; it catalogs beings and mineral wealth associated with watery/subterranean zones, a typical Vastuvidya-style description of what lies beneath or within certain regions.
It supports prudent governance and household prosperity by indicating where valuable resources (gems) may be found and by implying caution around dangerous aquatic life—useful for settlement planning, resource management, and protection of people.
In Vastu contexts, references to aquatic creatures and hidden gems function as site indicators: they suggest water presence and subterranean conditions, informing decisions about excavation, foundations, wells, and the suitability of land for building.