Aquarium staff were left mystified after a crate containing a giant eel but no water was left at their reception.
The five-foot-long conger eel was left in a blue crate in Tynemouth's Blue Reef Aquarium's reception by a member of the public.
The unknown person left before any member of staff could speak to them.
Despite being out of water for some time, the eel is said to be making a good recovery.
Blue Reef Aquarium's Anna Etchells said: "We simply have no idea why this person decided to bring the eel in to us.
"It may have been caught by someone out fishing or perhaps spotted stranded on the beach or in a rock pool but that still doesn't explain why they would bring it here and, more importantly, why it was out of the water.
"Conger eels are capable of surviving for some time out of water and amazingly it appears to have made a full recovery."
Congers are the largest eels in the world reaching lengths of up to three metres and weighing anything up to 110 kg.
During the day congers are found in holes or crevices on rocky or sandy bottoms and in wrecks and other artificial environments.
They become more active at night when they leave their shelters to go in search of prey.