i had my tickets bought before arriving in lisbon because this was the one attraction i could not miss. there is not much to be said, all the reviews highlight how beautiful, interesting, educative this place is. for sure you should make time to visit it, i think 2 hours are enough to make the tour while taking photos and admiring the tanks.
it's easy to get here with the metro and only 10 min walk distance from the metro station. if you want to buy the tickets from the internet you will pay an extra 2E i think per ticket. when i went there ( around 20th of august) there was only a small crowd waiting to buy tickets.
also there was a temporary exhibition with sea turtles, very beautiful again.
near the oceanario you can also see the longest bridge in europe, vasco da game bridge, and you can make some shopping at casco da gama shopping center - right next to the metro station, so you can give this place of lisbon half a day of your trip
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