Sage Kapila was residing in Rasatala. This is one of the seven worlds below the Bhu loka. The fourteen worlds are
From Valmiki Ramayana, Bala Kanda Sarga 40,
pitur vacanam āsādya sagarasya mahātmanaḥ |
ṣaṣṭiḥ putra sahasrāṇi rasātalam abhidravan || 1-40-12
On catching up the words of their father, the great-souled Sagara, his sixty thousand sons rushed towards the surface of the rasaa tala, the netherworld.
Sixty thousand sons of Kapila reach rasatala and dig the earth in different directions circumabulaitng the elephants which bear the quarters of the earth known as Ashtadiggajas. Finally, they see Kapila and also the sacrificial horse. They mistake him as the thief of the sacrificial horse and attack on him. Sage Kapila burns down them to ashes.
There is a brief description of this world in Srimad Bhagavatam Skandha 5 chapter 24
Beneath Mahātala is the planetary system known as Rasātala, which is the abode of the demoniac sons of Diti and Danu. They are called Paṇis, Nivāta-kavacas, Kāleyas and Hiraṇya-puravāsīs [those living in Hiraṇya-pura]. They are all enemies of the demigods, and they reside in holes like snakes. From birth they are extremely powerful and cruel, and although they are proud of their strength, they are always defeated by the Sudarśana cakra of the Supreme Personality of Godhead, who rules all the planetary systems. When a female messenger from Indra named Saramā chants a particular curse, the serpentine demons of Mahātala become very afraid of Indra.
It is said by Narada while searching a best groom for the daughter of Matali that it is one of the happiest places.
From Mahabharata Udyoga Parva
The couplet that was sung by the dwellers of Rasatala in day of old, is still heard to be recited in the world by the persons of learning. That couplet is this,--Neither in the region of the Nagas, nor in Swarga, nor in Vimana, nor in Tripishtapa is residence so happy as in Rasatala!'"
It is probable that other puranas also talk about Rasatala.