Under the treaty, they could send submarines through the straits but they had to notify Turkey: Under Article 12, Black Sea states are also allowed to send sub

Under the treaty, they could send submarines through the straits but they had to notify Turkey: Under Article 12, Black Sea states are also allowed to send submarines through the Straits, with prior notice, as long as the vessels have been constructed, purchased or sent for repair outside the Black Sea. The less restrictive rules applicable to Black Sea states were agreed as, effectively, a concession to the Soviet Union, the only Black Sea state other than Turkey with any significant number of capital ships or submarines.[8][14] The passage of civil aircraft between the Mediterranean and Black Seas is permitted, but only along routes authorised by the Turkish government.[15] When I sailed through the Bosphorous, we had a tour guide who told us that once a Soviet sub was spotted outside of the black sea and later spotted inside but notice had never been given and the sub had never been seen going through. They later figured out that the sub had made the journey by shadowing underneath a large civilian ship to prevent from being spotted (by sonar I suppose). After that, they decided to run chains across the straight underneath the depth of a normal ship. I'm not sure how true it is, but it makes a good story. :-)