Valmiki Ramayana Kishkindha Kanda Sarga 1 Shloka 117

राघव Rama, रावणः Ravana, पातालम् underworld, ततः thereafter, अभ्यधिकमेव वा or even to a lower region, गच्छति यदि if he goes, सर्वथा by all means, न भविष्यति तावत् he will not survive.

'Even if Ravana escapes into the nether world or into a world still lower, he cannot save himself by any means, O Rama