Agastyakuṇḍa (Agastyakunda) is a spring on Udyantaka Parvata (Udyantaka Hills). The spring is so called because sage Agastya had set up this mountain at this place. Eight sages, like Vyāsa (Vyasa) and Śuka (Shuka), had obtained salvation by performing tapasyā (tapasya) at this place. It is said that auspicious results may be obtained by one who performs an act of worship at this place, remembering those eight sages.
In this context it may be mentioned that multiple springs going by the name of Agastyakuṇḍa can be found at different places all over India, though scholars are of the opinion that at least one Agastyakuṇḍa lies in modern day Benares. [Encyclopaedia of the Hindu World,Vol 1. Ed. Ganga Ram Garg. p. 203]
The Agastyamalai Hills (also known as Pothiyar Hills) which stretches across the border between modern day Kerala and Tamil Nadu is believed to be the Udyantaka Parvata of ancient times by many. There is a spring called Agastyakuṇḍa on this Agastyamalai Hills as well.
However, according to the description in the Vāyupurāṇa (Vayupurana), the place where Agastya and the other eight saints had performed their tapasyā was Naimiṣāraṇya(Naimisharanya), and the hills on the left of this hermitage of the sages was Udyantaka Parvata. In that case it appears that both Agastyakuṇda and Udyantaka Parvata lie close to Naimiṣāraṇya, which is identified with the Nimsar or Nimkhar forest in modern day Uttar Pradesh. A verse in Garuḍapurāṇa (Garudapurana) says that the holy places of the Sarasvatī (Saraswati) River, Kurukṣetra (Kurukshetra) or Prabhāsakṣetra (Prabhas) may also be found at Gayākṣetra (Gaya in modern day Bihar). This kind of a reference makes it appear that Udyantaka Parvata, which lies near Naimiṣāraṇya, might have had its name applied to Gayākṣetra at a later period. This further points to the presence of an Agastyakuṇḍa in modern day Bihar.