Echoes of the Multiverse

I have looked in the manual on water pressure or general pressure, but nothing comes of it. How can I put it in a game? My players are in submarine base and they want to investigate the sea bottom.

From True 20:

As one goes deeper, water pressure (that is, the pressure exerted by the weight of the water around you on your body) increases. At a depth of 200 feet, the pressure exerted on the human body is almost 90 pounds per square inch. For every 100 feet below the surface, a character takes +2 impact damage every minute. This can be negated by a Difficulty 15 Fortitude save, +1 for each previous check. Objects are not immune to this damage unless they are solid all the way through.

Many (but not all) aquatic natives are immune to pressure, but sudden changes may cause damage. If a creature with the aquatic subtype travels upward or downward more than 200 feet without stopping for at least 1 minute every 100 feet to stabilize, it must make a Difficulty 15 Fortitude save or take +2 non-lethal damage. This affects surface dwellers as well, but they are less likely to linger at such depths.

Thanks, i have a more accurate idea.