Matsya Purana — The Cave-Sanctuary: Jewel-Lake
अनास्तृतगुहाशायी कालं नयति पार्थिवः त्यक्ताहारक्रियश्चैव केवलं तोयतो नृपः न तस्य ग्लानिमायाति शरीरं च तदद्भुतम् //
anāstṛtaguhāśāyī kālaṃ nayati pārthivaḥ tyaktāhārakriyaścaiva kevalaṃ toyato nṛpaḥ na tasya glānimāyāti śarīraṃ ca tadadbhutam //
The king passed his time lying in a cave without any bedding. Having abandoned food and all ordinary acts, he lived on water alone; no weariness came upon him, and his body became wondrously enduring.
This verse does not describe Pralaya; it focuses on tapas—how austerity can make the body steady and free from fatigue, highlighting inner mastery rather than cosmic dissolution.
It presents an ideal of royal self-restraint: a king can undertake disciplined austerity—reducing comforts, limiting intake to water, and giving up ordinary indulgences—to strengthen resolve and rule with detachment and steadiness.
There is no direct Vastu or temple-building rule here; the only implied practice is ascetic living in a cave without bedding, reflecting simplicity and renunciation rather than architectural prescription.