I was reading the biography of a 14th century Magadhan Buddhist monk (Sunyaadis'ya) who traveled all over India, Lanka, China and Korea before finally settling in Korea. The interesting thing is that he had studied and ordained in the famous Nalanda university, which was the center of Mahayana Buddhism in middle India at that, but he was sent by his professors to Sri Lanka to meet a famous master their to be able to understand the meaning of Prajna. This he did, and after spending 6 months in a cave in Ceylon, he finally understood the meaning of Prajna which that master had asked him to find.
To read the full bio click on this link: http://buddhism.lib.ntu.edu.tw/FULLTEXT ... waley1.htm
Both Vajrabodhi and his disciple Amoghavajra also traveled to Sri Lanka to learn and collect Tantric manuals (Mahavairocama sutra especially). So it appears that despite the present reputation of Sri Lanka as a Theravada center, Sri Lanka was well known in medieval and ancient history as a prominent location for Mahayana and Vajrayana Buddhism. Not to mention, even the Lankavatara Sutra is supposed to have been delivered in Sri Lanka.
How and why did Mahayana/Vajrayana become completely extinct in Sri Lanka?