I think they are sealed

[Zarathustra]

The batches of mead are in two 5 gallon "Pepsi" cannisters used for fountain drinks. All of the rubber seals on the cannisters were good 4 years ago. I shot some carbon dioxide in the cannisters when I first put the mead into them. The other day I a pulled up on a pressure release valve and pressurized air (carbon dioxide?) came out.

[fitz]

Sounds like you may have some sparkling mead.
Not necessarily a bad thing. If the mead kept some of its residual sweetness from the honey, it will be a nice honey champagne type drink. give it a shot. many homebrew stores are selling a picnic type tad for the Pepsi containers. They do not carbonate the liquid, it just pushes it out. Very similar to a co2 bike pump, but it doesn't have the oil like the bike canisters or airgun canisters will have.