Andre Norton 
Key Out of Time 

Far in the future, the planet Hawaika is a tropical paradise settled by a small group of descendants of modern-day Hawaiians and Polynesians. But Ross Murdock and Gordon Ashe are not there to languish on the planet’s beautiful beaches — they want to establish a portal to the past. Will they succeed in their quest to find out what became of the ancient civilizations that once thrived on Hawaika?