How can I mitigate high humidity in a safe?

In the most downstairs portion of our house we have a waterproof fire safe with standard important documents in it. For some reason, during the summer only, it seems like humidity builds up within the safe and specifically causes the passports to start curling. We don't see any kind of curling with any of the other documents stored in the safe, just the passports.

I am unsure what kind of coating is used on the passports, but I'm sure it's got a bajillion security features. Regardless, I'm not sure if those same features are contributing to the curling.

Thus far, we have tried collecting silicon packets from new pairs of shoes and putting them in the safe. That has not fully alleviated the problem and I want to know if just going for a full bowl of rice will solve the problem? My concern with this latter solution is that the space inside the safe isn't very big, I'd like to avoid eating up a bunch of it with a rice bowl.

In case it's relevant, we have a smart thermostat that strives to maintain a temperature inside the house of around 82 F during the day when nobody's in the house and keep the humidity levels between 40-55%. Depending on how bad the outside humidity is, this isn't always feasible, but I'd say the humidity in the house never exceeds 60%.

Does anyone have any ideas for how to dramatically lower the humidity within a small, enclosed space like this? Or should I just fully commit to rice?

"Downstairs" and "in the summer" mean:

• A cool location.
• Hot humid air.

It sounds like:

• The safe is not hermetically sealed, or is frequently opened.
• The safe is sometimes cooler than the surrounding air.
  (Does it ever form condensation on the outside?)

So, under some circumstances the outside humid air enters the safe and then the humidity condenses on its cooler interior.

It is unlikely that the safe is ever significantly warmer than the surrounding air, so the reverse process doesn't happen.

The result is a form of Atmospheric water generator, a device used in locations lacking any water supply to extract drinking water from the air. The safe is slowly pumping water from the surrounding air into itself.

You can't do much about the first point, but you could add a small heater to the safe to keep its interior slightly warmer than the surrounding air.
Depending upon the size of the safe, even standing it on a heating pad for plants might be enough.

Or even simpler, perhaps the safe is sitting on a cold basement floor and simply putting a layer of insulation under it will do the trick.