http://www.cnn.com/2017/07/31/politics/north-korea-ejection-test-submarine-activity/index.html
https://www.google.com/amp/mobile.reuters.com/article/amp/idUSKBN1AG04F
And for some perspective from the Congressuonal Research Service (CRS):
https://fas.org/sgp/crs/nuke/R41259.pdf
and
And