Is my snail dead?

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We have two African Land Snails. I think one of them is dead, but how do I tell? Its retracted its self really far into its shell.

Any idea what to do?

Dead snails really, really stink. Take them out and give them a sniff? To be fair you probably wouldn't need to get them out, just take the lid off the tank and the smell would be obvious.
If the snail is really far up in the shell and has sealed itself in with a whitish layer of mucus, it's probably just not humid enough for it. Spray loads of water in with a mister and see if they wake up
I hope the snails are OK!