According to tradition, the Mahayana sutras were retrieved from the naga realm, when the world was ready to receive them. This might seem farfetched, but the nagas act as Dharma protectors in the Pali canon: http://www.accesstoinsight.org/tipitaka/kn/ud/ud.2.01.than.html
Contemporary Buddhist scholars tend to speculate the Mahayana sutras were received in states of meditative samadhi. How can we know that these meditation masters, in profound samadhi states, weren’t accessing previously unpublished sutras from the naga realm?
