Valmiki Ramayana 6.22.37

तस्माद्बाणमुखात्तोयमुत्पपात रसातलात् ।स बभूव तदा कूपो व्रण इत्येव विश्रुतः॥

tasmādbāṇamukhāttoyamutpapāta rasātalāt |sa babhūva tadā kūpo vraṇa ityeva viśrutaḥ ||

Our plain-language meaning of this verse is waiting for scholar review.