Skandapurana 1.3.7.87

1.3.7.87

kṣīrābdhermathyamānasya parvato hi rasātalam..
gataḥ sa tatkṣaṇādeva kūrmo bhūtvā ramāpatiḥ..
uddhṛtastatkṣaṇādeva tadadbhutamivābhavat.. 9.87 ..

As the ocean of milk was being churned, the mountain sank to Rasātala; / but becoming a tortoise at that very instant, the Lord of Ramā / lifted it immediately, which was like a wonder.