The strange submarine was found Friday morning in the East River near Red Hook, Brooklyn.
One man was inside and two others were in an inflatable boat nearby.
Police said they aren't sure what the men were up to but there was no indication the incident was part of a terrorist plot.
Right now they're classifying the incident as "marine mischief."
Investigators said the vessel was partly submerged and may have been designed for underwater navigation.
The three men are expected to be charged in the incident.
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