The absence of human sounds was mesmerizing. Insects buzzed, and birds sang, their sounds rising and falling as they drew near and flew off. One sound we didn't hear was the tell-tale drumming on tree bark of Ivory-billed woodpeckers, though some think the birds thought extinct might exist deep within Congaree National Park in South Carolina.

As beautiful as above-ground landscapes are in the National Park System, if you head underground you'll find stunning caves that will leave you breathless. Carlsbad Caverns National Park in New Mexico is, arguably, the biggest cave draw in the park system, and perhaps the most beautiful.

Southern Utah's national parks all are red-rock wonders. Zion National Park features towering cliffs of stone. Arches National Park showcases a one-of-its-kind collection of stony arches and windows. Canyonlands National Park is a maze of canyons, and Capitol Reef National Park offers a sprawling rockscape. Bryce Canyon, on the other hand, is much, much more intimate and, in some manners, more curious, geologically.

A subtropical wilderness surrounded by urbanized Florida, Biscayne National Park is a tantalizing water world inviting you to get wet.