Zcash is preparing to shut down its oldest privacy technology, potentially leaving millions of dollars in cryptocurrency inaccessible to their owners—even if they still have the passwords and private keys needed to access their wallets.

Approximately 12,224 ZEC remain in Sprout, the cryptocurrency’s original privacy pool, according to ZecStats data updated Thursday.

At recent market prices, those holdings represent roughly $15 million to $16 million that could become effectively frozen when the network activates its NU7 upgrade.

“NU7 (block 3,543,405) makes v4 transactions invalid, so what remains in Sprout is unspendable: frozen, not burned,” Zecstats says in its information card for Sprout.

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