https://en.wikipedia.org/wiki/Amphora might be an option.

I like the lead idea too...

But if I were going to do it -
Take all the documentation.Wrap in layers of cloth and leather.
Sew it up in a thick heavy leather bag, with natural glue on the
outside of the seams.
Paint the whole thing in wax until it's got about 3mm thick walls.
Carve in instructions on the wax in all available languages that this
is a document cache and that it should be carefully cut open on the
dotted lines.
Pack the thing in a sand filled container, and use lost wax casting to
replace the wax with tin. Melting point is 449F, so we're below the
mythical Fahrenheit 451 and the multiple layers of insulation, so the
documents should be fine. I was originally going to go with this step
in lead, but tin has a lower melting point. (449F is well within
tolerance for an IISS hand computer rated for EVA use, so this isn't
going to break that)
Dig it out of the sand, do a little pick and file work on the
instructions to clean up casting flaws.
Ba-da-bing, time capsule.

On Sun, Dec 8, 2019 at 6:56 PM Kurt Feltenberger <xxxxxx@thepaw.org> wrote:
>
> I'm wondering how someone in the late 16th to early 18th century would
> seal a container to make it waterproof and also protect it from the
> ravages of being submerged in seawater for several hundred years.  I'm
> thinking it could be a regular wooden chest, but then wrapped in lead
> sheets with all the edges sealed with molten lead.
>
> The concept is that the characters are on vacation to a world that just
> created the jump drive and by all observable data points, they did it on
> their own and without outside assistance.  While cruising across a
> tropical sea, they stop at a shoal to do some recreational diving and
> find an old wreck.  Inside the wreck is the sealed container and when
> they open it, they find several repair logs, a couple text books on jump
> drive design and theory, a log book, and a small computer with a
> holographic display.  The twist is that the wreck is dated from almost
> four hundred years earlier, the books and computer are from slightly
> before that, and the log book is from an IISS ship that disappeared
> about the same time.
>
> The characters, being characters, begin researching what they found and
> put the planet's status as another major race of Humanity at risk...and
> thus targets on their backs.
>
> Any thoughts?
>
> --
> Kurt Feltenberger
> xxxxxx@thepaw.org/xxxxxx@yahoo.com
> “Before today, I was scared to live, after today, I'm scared I'm not living enough." - Me
>
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