Canto 2 -
Ayodhyā-kāṇḍa
Chapter 77: Bharata and Śatrughna Lament About Daśaratha’s Departure
Text 2.77.13

मन्थराप्रभवस्तीव्रः कैकेयीग्राहसङ्कुलः।
वरदानमयोऽक्षोभ्योऽमज्जयच्छोकसागरः॥

mantharā-prabhavas tīvraḥ kaikeyī-grāha-saṅkulaḥ
vara-dānamayo ’kṣobhyo ’majjayac choka-sāgaraḥ

mantharā-prabhavaḥ = it has originated from Mantharā; tīvraḥ = and terrible; kaikeyī-grāha-saṅkulaḥ = and filled with a crocodile in the form of Kaikeyī; vara-dānamayaḥ = in the form of your bestowal of the boons; akṣobhyaḥ = an imperturbable; amajjayat = has drowned us [now]; śoka-sāgaraḥ = ocean of misery.

An imperturbable and terrible ocean of misery in the form of your bestowal of the boons has drowned us [now]. It has originated from Mantharā and filled with a crocodile in the form of Kaikeyī.

This ocean came from Mantharā, not King Sagara. It was terrible, that is, unfathomed.