[King Sagara had two wives, Sumati and Keśinī.] The sons of Sumati, who were very proud of their prowess and influence, following the order of their father, searched for the lost horse. While doing so, they dug into the earth very extensively.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
और्वोपदिष्टयोगेन हरिमात्मानमीश्वरम् । तस्योत्सृष्टं पशुं यज्ञे जहाराश्वं पुरन्दरः
aurvopadiṣṭayogena harimātmānamīśvaram | tasyotsṛṣṭaṃ paśuṃ yajñe jahārāśvaṃ purandaraḥ
SB 9.8.8