One out of every ____1____ humans lives above karst formations. Karst caves are the result of soluble rock dissolving away with the power of groundwater. Not all rocks are durable, some are easily eroded. ____2____, marble and gypsum are examples of soluble rock types that can weather away to form caves. As surface water enters the ground, it flows through cracks and holes in the bedrock. As water flows through karst landscapes, caves are carved out of stone. Karst landscapes hold about ____3____% of groundwater in the United States. Karst is central to the origin of the largest caves in the world, and many scenic attractions worldwide feature the beauty of this geologic formation.

The Formation of Karst Caves

As rain falls through the atmosphere, water droplets pick up molecules of carbon dioxide that impart a slight acidity to the water. This acidity slowly dissolves the ____4____ bedrock of karst. When fractures in the bedrock open up to the surface and become large enough to explore, they become considered _____5_____. The largest caves in the world are the result of this weathering process in karst geology.

The easy passage of water through karst formations has benefits and disadvantages. Water can be stored and made available quickly for people and ____6_____, but the same characteristics of rapid percolation mean that few ____7____ are filtered from the water as it enters the aquifer. Aquifers are underground rock formations that hold water. Over 20% of all groundwater used by cities and farmers in the US comes from karst aquifers. 

Threats

Because of the rapid percolation of water into karst aquifers, groundwater pollutants are especially impactful. Pollution from industry, agriculture and mining threatens the water sources used by millions of people. In some karst systems, unusually adapted creatures survive in the karst caves. In Arkansas and Missouri, the blind Ozark cavefish (scientific name Amblyopsis rosae) has lost the ability to see clearly after many generations away from ____8____. For most creatures, genetic mutations that cause poor eyesight would be detrimental to the survival of that individual, but not in the dark waters of a karst cave. Polluted groundwater and low water levels due to drought and water extraction threaten the survival of this species and others like it. 

Noteworthy Karst Caves

Mammoth Cave National Park in ____9____ is the world’s longest cave system. ____10____ miles of connected passageways exist in this karst cave that may go much further than currently known. Mammoth Cave is considered a solution cave. These types of caves form when rainfall filters through the soil and into the bedrock, where it becomes slightly acidic with the addition of carbon dioxide in the soil and rock. This acid weathered away the bedrock under this national park for millions of years to form many miles of caves that are open to the public today. 

The Son Doong Cave in ____11____ is the largest single cave in the world. Unlike Mammoth Cave which is made of many smaller connecting caves, Son Doong is large enough to fit an entire ____12____ inside with ease. This karst cave was formed by soluble limestone between 2 and 5 million years ago. It was discovered accidentally by a local in 1994 and has since become a popular tourist attraction.

Waitmo Glow Worm Cave in ____13____ is a naturally illuminated karst cave. Similar to fireflies, the larvae of the fungus gnat (scientific name Arachnocampus luminosa) produce bioluminesce, or naturally-produced light. The bluish glow of their ____14____ lights up caves in a magical way. Imagine the awe of a massive cave with glowing walls around you. Scientists think that the bioluminesce attracts prey for the larvae to feed on. 

Sac Actun is the longest underwater cave in the world. This karst cave network is located in the Mayan heartlands of the Yucatan Peninsula in Mexico. Its unique name means ‘____15____’ in the Yucatec Mayan language. But Sac Actun is not alone in the region; hundreds of other underwater caves are known to exist in the Yucatan. Sac Actun and other huge caves nearby often take turns claiming the title of the world’s longest cave as new connections are discovered, adding to their official length. 

Get Out and Experience the Wonders of Karst Caves

All of these famous caves and many more are open to the public for exploration. Park rangers safely guide visitors into some of the most beautiful geologic formations on Earth. Plus, there is more to discover as the possibilities are endless with underground adventure.