Holiday World (@holidayworld) is reporting on Twitter that the park is being evacuated due to a "threat." "All are safe." Link to tweet:
 

Following a threat at the park this evening, we are safely evacuating all Guests and Employees in cooperation with @ISPJasper All are safe.

If you are at Holiday World, stay safe. Follow all instruction from Holiday World and the Indiana state police.
 
Comments on Facebook are suggesting that there was a bomb threat. Holiday World replied to another Twitter user and said that there was an "unsecured backpack."
 
EDIT: The park has posted on Facebook:
 

Following the discovery of an unattended backpack, we are working with State Police to safely evacuate the park of Guests and Employees. We apologize for the inconvenience and are providing return tickets to the evacuated Guests.

What sad times these are, when an unattended backpack can instill fear, and interrupt so much in and of our lives. Glad everyone is safe.  

Park says that a guest turned in a suspicious backpack. Link:
 

Park guest turned in what he believed to be a suspicious, unattended backpack.

Indiana state police are reporting that the scene is now all clear. Link:
 

UPDATE
The backpack at @HolidayWorld is ALL CLEAR, the scene is secure.

There's an important message here about how invaluable it is that park security is what it is these days. I absolutely agree with Thrill_Biscuit that it is sad that an unattended backpack is a cause for alarm, but it also makes me appreciative that parks have responses prepared for these situations, including bag checks and metal detectors. Stuff like this has the potential to be a lot worse without the security that parks put in place.

Just think... If someone did this at Banshee and Kings Island had to evacuate because of the no-loose article policy...

Sad this is what it's come to. When I was a kid you would've never heard of such a thing........

Wasn't too long ago that both of Disneyland's parks were shut due to a suspicious metal object in a tree in the Esplanade between the parks.

 

It was a geocache.

 

Better safe than sorry.