I am planning an underwater cache, scuba accessed (35 feet deep - so that it is accessable for even Junior Open Water divers - like my daughter). I figure that Okinawa has great coral, and the coral has got cracks, so a rock face climbing cam (or piton) should hold whatever container I decide to use on the bottom (just have to find a way to lock the cam so it can't be removed - easily). I plan to use a stainless steel combination lock to attach the cache to the cam, to prevent non-geocaching scuba divers from muggling the cache. The combo will be a significant year in Okinawan history...
I do have to be careful not to make the cache look like UXO - Unexpoded Ordinance from WWII or I can count on having the cache muggled by EOD Techs - Explosive Ordinance Disposal Technicians.
Does anyone know if the self-purging valve on a Pelican Container is a one way valve? I.e. lets air out but not back in? How deep will one survive? - just checked their website, they say will survive to 50 feet.
Naturally I have a bit of planning to do:
I have to find a dive site,
find a cache site on that dive site (where the bottom is 35 feet deep),
get the GPS coordinates at the surface directly above the site,
determine what container will survive,
determine whether the lock will survive the saltwater,
determine how much I'm willing to spend on a cache, cams aren't cheap nor are Pelican Containers,
figure out how to lock the cam so it can't be removed from the bottom,
figure out how to prevent the cache from being opened underwater or on the surface, or
make the cache water-filled as someone mentioned,
figure out how to make the process easy enough so someone will put it back for others to enjoy,
find swag and FTF appropriate to Davy Jones' locker,
lastly emplace and post the cache!
***Disclaimer***
Of course, if any of this requires damaging the fragile underwater ecosystem, it's not worth the work. I will not destroy in order to create.