The Dharma Vāpī is a very holy vāpī (tank) in Dharmāraṇya. Dharma, in the form of a bull, made it on the word of Bhagavān Śiva. Bhagavān Śiva had appeared before Dharma after thirty thousand years of his tapas (austerity). The tank reaches from Darbhaśayana to Devīpattana and is the ritual heart of the Dharmāraṇya kshetra. Every act done on its banks (japa, homa, dāna, piṇḍadāna and śrāddha) gives ananta-phala, fruit that has no end. The Dharmeśvara Liṅga of Bhagavān Śiva was set up on its banks.

Origin

Dharma did tapas for thirty thousand years in the forest that was to become Dharmāraṇya. Then Bhagavān Śiva appeared before him in blazing splendour. Dharma praised him with a stotra (hymn of praise) of forty names. Bhagavān Śiva gave the place the name Dharmāraṇya. He told Dharma to make a sacred tank there, for the good of all who in future would seek dharma and moksha.

Dharma made the tank, the Dharma Vāpī. He stretched it from Darbhaśayana to Devīpattana, so that it held the whole extent of the kshetra. Then Bhagavān Śiva set up the Dharmeśvara Liṅga on its banks, as his own lasting presence. (Skanda Mahā Purāṇa, Brahma Khaṇḍa, Dharmāraṇya Khaṇḍa, Pūrvārdha, Chapter 4)

Holy uses

The Dharma Vāpī is the ritual heart of Dharmāraṇya, used for many holy purposes.

Piṇḍadāna and śrāddha for the ancestors on its banks give ananta-phala. The pitṛs reach tṛpti (full satisfaction) and rise to svarga.
Whatever japa, homa or svādhyāya is done here, its fruit never runs out.
A ritual bath in the Dharma Vāpī destroys all the pāpa (sin) a person has gathered, leaving none behind.

(Skanda Mahā Purāṇa, Brahma Khaṇḍa, Dharmāraṇya Khaṇḍa, Pūrvārdha, Chapter 4)

Its glory in all the yugas

The glory of the Dharma Vāpī stays the same in all four yugas. Even in Kali Yuga, when most holy places have lost their power, the Dharma Vāpī keeps its full power to purify. One act of piṇḍadāna or tarpaṇa done here equals thousands of such acts done elsewhere. The Dharma Vāpī is said to be especially good for those troubled by the debt to their ancestors (pitṛ-ṛṇa), because it is the easiest way to pay this debt in the present age. (Skanda Mahā Purāṇa, Brahma Khaṇḍa, Dharmāraṇya Khaṇḍa, Pūrvārdha, Chapter 7)

The Dharma Pushkariṇī of the Setu-kshetra

The Setu Māhātmya of the Skanda Purāṇa describes a separate Dharma Pushkariṇī at Mount Gandhamādana, in the Setu-kshetra on the shore of the southern ocean. Dharma also made it, and the Sudarśana cakra took up its lasting home there.

Both the Dharma Vāpī of Dharmāraṇya and the Dharma Pushkariṇī of the Setu-kshetra are sacred waters made by Dharma. Each was made in a different kshetra, as a result of Dharma’s tapas and of the vara (boon) of Bhagavān Śiva or of Bhagavān Vishṇu. Each is said to reach through the whole of its kshetra. (Skanda Purāṇa, Setu Māhātmya)

Sources

Skanda Mahā Purāṇa, Brahma Khaṇḍa, Dharmāraṇya Khaṇḍa, Pūrvārdha, Chapters 4, 7
Skanda Purāṇa, Setu Māhātmya