What Went Down is one of seven mini games based on the d20 system, requiring the 3.0 Dungeons & Dragons Player's Handbook. From the publisher's website:

... something ... gone ... movement ... where are you ...

UNDER ICE

You know the ocean. You’ve been pulling people from it for years.

This is different. A Russian submarine is beneath the Arctic, in the remotest water on the planet. For its crew - you are the only hope.

Deep ... desperate ... deadly
... Fright Night

DAVEY JONES’ LOCKER

There are unanswered questions here.
Why did the sub go down?
Is there a hidden agenda?
And the crew: Where are they?

FRIGHT NIGHT: WHAT WENT DOWN:

• 6 new character classes including Marine Biologist, Rescue Diver, and Salvage Operator
  
• Character backgrounds such as Oilrig and Navy
  
• 16 new skills including Scuba and Underwater Hazard
  
• Over 25 new feats such as Bright Idea, Swim like a Fish and Tech Specialist
  
• 2 fully detailed new submersibles: the Erebus rescue sub and the Russian nuclear attack sub Koloss
  
• Ideas for expanding the horror of WHAT WENT DOWN into a full underwater salvage campaign

Everything you need for terror with depth