When in Seattle, it means the Seattle Underground. Pioneer Square was the center of Seattle in the mid-19th Century. The area was composed of wooden structures. It was built on filled in tide lands and was often flooded. In 1889 the area was destroyed by fire. Pioneer Square was rebuilt in brick and stone and regraded 1-2 stories above the original street plan. What is left are a series of underground tunnels and the basements of the original businesses.  A small part of the underground was restored and in 1965 the Underground Tours were started. Visitors and go underground and learn about the history of this historic and colorful area.

http://www.undergroundtour.com

Ape Caves, located in Gifford Pinchot National Forest south of Mt. Saint Helens National Monument. The caves include the longest lava tube in the continental US. Allow 4 hours to explore the area.

The Cheese Caves are also located in Gifford Pinchot National Forest. Here the Guler Cheese Company stored their prize-winning cheeses deep in the lava tubes. Start next to the original cheese factory and head down to see remnants of where the cheese was stored.

Gardner Cave, is located in Crawford State Park, north of Spokane. This 1000 ft. limestone cave includes stalactites and stalagmites. The cave is open May through September and has a 40-minute guided tour.

Boulder Cave is tucked away in the Wenatchee National Forest. This basalt cave was carved between two layers of volcanic rock by Devils Creek.  The creek still flows though the cave. Boulder Cave is open spring and summer and is home to big-eared bats