The picture in my blog header shows the man-made cave used for the Sixth Buddhist Council.

Soon after regaining Independence, the Government of Myanmar began plans to hold a Sixth Buddhist Council (Chaṭṭha  Saṅgāyana) in Myanmar, with four other Theravāda Buddhist countries (Sri Lanka, Thailand, Cambodia, and Laos) participating. For this purpose the Government dispatched a mission to Thailand and Cambodia, composed of Nyaungyan Sayādaw, Mahāsi Sayādaw, and two laymen. The mission discussed the plan with leading Buddhist monks of those two countries.

In the historic Sixth Buddhist Council, which was inaugurated with every pomp and ceremony on 17th May 1954, Mahāsi Sayādaw played an eminent role, undertaking the exacting and onerous tasks of Final Editor (Osana) and Questioner (Pucchaka). A unique feature of this Council was the editing of the commentaries (Aṭṭhakathā) and Subcommentaries (Ṭīkā), as well as the canonical texts. In the editing of this commentarial literature, Mahāsi Sayādaw was responsible for making a critical analysis, sound interpretation, and skilful reconciliation of several crucial, but divergent passages.

A significant result of the Sixth Buddhist Council was the revival of interest in Theravāda Buddhism among Mahāyāna Buddhists. In 1955, while the Council was in progress, twelve Japanese monks and a Japanese laywoman arrived in Myanmar to study Theravāda Buddhism. The monks were initiated into the Theravāda Buddhist Saṅgha as novices while the laywoman was made a Buddhist nun. Then, in July 1957, at the instance of the Buddhist Association of Moji, the Buddha Sāsana Council of Myanmar sent a Theravāda Buddhist mission to Japan. Mahāsi Sayādaw was one of the leading representatives of the Burmese Saṅgha in that mission.

Also in 1957, Mahāsi Sayādaw undertook the task of writing an introduction in Pāli to the Visuddhimagga Atthakathā, to refute certain misstatements about its famous author, Ven. Buddhaghosa. The Sayādaw completed this difficult task in 1960, his work bearing every mark of distinctive learning and depth of understanding. By then the Sayādaw had also completed two volumes (out of four) of his Burmese translation of this famous commentary and classic work on Buddhist meditation.