Muñjavān, also spelled Mañjavān, Muñjavat and Muñjavanta, is a sacred mountain in the north. The Ṛgveda names it as the home of Rudra and the source of Soma. The Vāyuvīya Saṃhitā tells that Brahmā did intense tapas (austerity) here with the pañcākshara (the five-syllable mantra of Śiva) as his only practice, and that he reached Śiva-sāyujya, union with Bhagavān Śiva. The Mahābhārata places Muñjavanta on the side of Himavat as the eternal home of Bhagavān Umāpati (Śiva), and tells of the gold hidden there.

The Vedic reference

Muñjavān is one of the oldest sacred mountain names in the Vedic texts. The Ṛgveda, and the Atharvaveda, name Muñjavān as the special home of Rudra, a mountain in the far north where Rudra lives in his most powerful form. Soma, the sacred plant of the yajña, grows on its slopes. Because the Veda ties the mountain to Rudra, the Purāṇas treat it as a natural place for Śaiva tapas.

Brahmā’s tapas here

The Vāyuvīya Saṃhitā gives the line of teachers of the pañcākshara in its pañcākshara-māhātmya section (Uttarakhāṇḍa, Chapter 12).

Bhagavān Śiva first gave the pañcākshara to Brahmā and to Brahmā’s four sons, Sanaka, Sanandana, Sanātana and Sanatkumāra. Brahmā was overcome by the power of the mantra and by thanks to Śiva. He went to Muñjavān Parvata and did intense tapas. His practice was simple and complete:

Place: Muñjavān Parvata.
Method: japa (repeating the mantra) of the pañcākshara without a break.
Food: not stated.
Length: long, until Śiva was pleased.

At the end of this tapas Brahmā reached Śiva-sāyujya, the state of complete union with the consciousness of Bhagavān Śiva. This is the highest fruit of the pañcākshara. Brahmā gained it by the mantra alone, without long rites, without a yajña, and without using his powers of creation.

What this shows

Even Brahmā, the highest creator, needs the pañcākshara for moksha (liberation).
Simple japa on a sacred mountain brings more than all of Brahmā’s other powers.
Muñjavān is a place where Bhagavān Śiva is directly present, as the Veda also says when it calls the mountain Rudra’s home.

Where it lies

The place of Muñjavān Parvata is ancient but not fixed with certainty. It is placed in the north-western Himālaya, perhaps in the region of present-day Kashmir or Himachal Pradesh. The Ṛgveda calls it Rudra’s home in the north, and the Atharvaveda names it in connection with Soma plants. Both point to a high Himālayan place.

Muñjavanta in the Aśvamedhika Parva: the hidden gold

The Aśvamedhika Parva of the Mahābhārata, in the Saṃvarta–Maruttīya ākhyāna (the account of Saṃvarta and Marutta), places Muñjavanta on the side of the mountain Himavat. It is the eternal home of Bhagavān Umāpati (Śiva). He lives there with Umā Devī among the roots of great trees, the mountain peaks, the uneven ground and the caves of the king of mountains. He carries the trident, he is full of radiance, and the hosts of bhūtagaṇas are always around him.

Also present are the Rudras, the Sādhyas, the Viśvas, the Vasus, Yama, Varuṇa, Kubera and his gaṇas, the bhūtas, the piśācas, the Aśvins, the gandharvas, the apsarasas, the Ādityas and the Maruts. Kubera’s own attendants, strange in form, play there while Bhagavān Śiva watches them with delight. He blazes with a splendour that ordinary eyes (carma-cakshus) cannot judge.

The mountainside is free of heat, cold, wind, hunger, thirst, old age, death and fear. Its stones scatter colours like the rays of the sun. Kubera’s armed attendants guard the whole region.

It was at Muñjavanta that the ṛshi Saṃvarta told King Marutta to recite a great Śiva stotra of a hundred names. This was the way to take safely the great hoard of gold (suvarṇa) hidden on the mountain. Marutta’s men brought back so much gold that his craftsmen made thousands of vessels from it for his mahāyajña. The wealth was so great that it caused Bṛhaspati envious grief and, in the end, Indra’s own challenge to Marutta.

Later, within the epic itself, Vyāsa sent Yudhishṭhira to this same gold left at Muñjavanta, to pay for his Aśvamedha.

The Mahābhārata tells this in Aśvamedhika Parva, Chapter 8.

Sources

Vāyuvīya Saṃhitā of the Śiva Purāṇa, Uttarakhāṇḍa, Chapter 12
Ṛgveda and Atharvaveda (the mountain named as the home of Rudra and the place of Soma)
Mahābhārata, Aśvamedhika Parva, Chapter 8