Just to follow on from Robert's post about the likelihood of submarines colliding. This is actually much higher than one might imagine. If  Starshiny Nemo, the webmaster of an internet site specialised in Soviet and Russian submarines, is right, the average works out at one collision every three years for those involving at least one Russian or Soviet sub (and during the Cold War this generally involved a US sub as well of course!), a figure which rises to once every 28 months if you count collisions with other objects (icebergs, oil platforms etc.) His site (in French only) http://pagesperso-orange.fr/souMarchov/  is a mine of information about Russian subs.

Between October and December 1967 collision of K-52 project 627a November while diving with the USS Madison (DD-425) in the Atlantic or Mediterranean;
15 November 1969 collision between the K-19 (knicknamed "Hiroshima" by Soviet submariners) project 658M Hotel II and the 615 GATO while diving;
20 July 1970 collision between the K-108 project 675 Echo II and the  USS Tautog (SSN-639) in the Sea of Okhotsk;
19 June 1972 collision of K131 project 675 Echo II with the K-320 project 670 Charlie I in the White Sea;
21 August 1977 collision of B-41 project 641 Foxtrott while diving and an oil platform in the Mediterranean;
13 April 1978 collision between the K-308 project 670 Charlie I and the K-490 project 667BDR Delta III;
10 July (June ?) 1980 collision of B-133 project 641 Foxtrott with the B-88 project 611 Zulu on the surface;
20 March 1981 collision of K184 project 675MK Echo II with the K-43 project 670 Charlie I on the surface;
23 May 1981 collision of K-211 project 667BDR Delta III diving with an unidentified foreign submarine;
21 January 1983 collision of K-10 project 675 Echo II with a Chinese sub in the Pacific;
22 October 1984 collision of K-373 project 705 Alfa with the K-140 project 667AM Yankee II on the surface in the White Sea;
30 October 1986 collision of K-279 project 667B Delta I with the USS Augusta (SSN-710), diving;
1 January 1987 collision of TK-12 project 941 Typhoon with the HMS Splendid (S106), diving;
11 February 1992 collision of K-276 project 945 Sierra I with the USS Baton Rouge (SSN-689);
20 March 1993 collision of K-407 project 667 BDRM Delta IV with the  USS Grayling (SSN-646), diving in the Barents Sea.
Between August and November 1979 the collision of K-513 project 671RT Victor 2 with an iceberg diving in the Arctic Ocean;
13 September 1984 collision of K-279 project 667B Delta 1 with the base of an iceberg 197m down;
19 May 1988 collision of K-475 project 667B Delta I with an iceberg 120m down in the Arctic Ocean.