Nobody really knows which portal leads to where. You'll always end up in a different spot in another hill, so it's easy to get lost! It's best to not mess around with the hills that are just mysteriously there without any trace of how they came to exist.
Just go to the beach instead, stay at one of the resorts and buy some souvenirs! It's never really warm though, which is why even the beach doesn't attract many tourists. It's a good spot for surfing and diving though. The brave ones try camping as well.