Arthur Curry: My parents were of two different worlds, and I was a product of the love that they shared. A son of land, and a son of the seas.
Orm: You were the reason our mother was executed. And I've hated you for it ever since. But I do not want to kill you, Arthur. I am going to give you one chance. Go home. Do not ever come back to Atlantis. You are not going to win this. A war is coming to the surface whether you like or not. And I am bringing the wrath of the Seven Seas with me.
Arthur Curry: You know I can't let that happen.
Orm: I know.
Jarhead: You the Fish Boy from the TV?
Arthur Curry: It's Fish Man.
Queen Atlanna: I am Atlanna, Queen of Atlantis.
Thomas Curry: I am Thomas, Keeper of Lighthouse.
Queen Atlanna: A king fights only for his own nation. A hero fights for everyone. You fight for everyone.
Mera: You think you're unworthy to lead because you're of two different worlds? But that is exactly why you ARE worthy!
Orm: My brother has come from the surface... to challenge me for the throne!
Arthur Curry: I call it an ass-whooping.
Arthur Curry: My father was a lighthouse keeper. My mother was a queen. But life has a way of bringing people together. They made me what I am.
Mera: Your half-brother, King Orm, is about to declare war upon the surface world. The only way to stop his war is for you to take your rightful place as King.
Arthur Curry: Trust me, I am no king.
Arthur Curry: I'm no leader. I'm not a king.
Mera: Atlantis has always had a king. Now it needs something more.
Arthur Curry: Well, what could be greater than a king?
Mera: A hero.
Arthur Curry: I'm no leader. I came because I have no choice. I came to save my home, and the people that I love.
Answer: You said it yourself some might have stumbled upon it, others could have used the map, which is why it was hidden in the first place.
How could they use the map when Arthur was the first to find it...quite frankly to stumble on it is impossible, you would have to randomly travel to the trench, go through the entire kingdom, see a tornado of death and decide to go in, get spat out and not eaten, get to shore, check behind a waterfall and follow it down...bit much?