On Tour

Your class will walk roughly one mile on the Adventure Tour, which lasts one hour. Your guide will stop the group roughly thirteen times on your trek through the cave in order to explain what can be seen and what we believe took place here prior to the cave’s discovery in 1963.

The tour begins in the pre-call room of the main building. Your guide will greet you and discuss the rules with your class before heading down into the cave. Please make sure everyone has used the restroom and that their shoes are tied!

As your class moves through the cave, the guide will be turning the lights in front of you on and the lights behind you off. This is partially to give the bats as natural a habitat as possible, but is also for communication purposes. When a guide comes to a room and the light is off, she has a better idea of where the guide in front of her is. If she comes to a room and the light is on, she will flash them once to let the other guide know that she is entering. Occasionally the lights will flash on your class as well; explaining this beforehand helps prevent the students from getting too excited when it happens.

Intro Room
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This room is used as our photo room. We will give each teacher a copy of her class picture when we exit the cave.

Soda Straw Balcony
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How our formations grow:

Ground water drips through the rock above our heads. As it interacts with decaying plant and animal matter, it picks up carbon dioxide (CO2). The result is a very weak acid called carbonic acid (H2CO3). (Acid rain is the very same solution, only it picks up its CO2 in the air.)

As this acid makes its way through limestone, it brings the mineral calcite (CaCO3) along with it. It is the pressure of the rock on this calcium bicarbonate solution that keeps it all together. When this drop of water, carbon dioxide, and calcite is no longer under pressure; say, when it reaches a void in the rock such as a cave, the solution is no longer stable, and the carbon dioxide evaporates. Since the drop of water is no longer acidic, it does not have the ability to hold on to the calcite. The calcite is deposited on the nearest surface, and the location of this surface is what determines the name of the formation that will eventually grow there.

Types of formations:
  • Soda straws are hollow, just like a straw
  • Stalactites hang tight to the ceiling
  • Stalagmites might reach the ceiling someday
  • Columns are when stalactites and stalagmites grow together
  • Flow stones form over walls, floors, boulders, and other formations
It takes roughly 100 years for one cubic inch of calcite to grow.

Temptation Stone
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When we touch a surface of the cave, we are depositing oils and dirt from our skin onto the rock. Oil and water do not mix, so when the water containing the calcite reaches this part of the cave, it will slide right over the spot that has been touched without depositing the calcite. The calcite is no longer able to “stick” to the formation, which means the formation stops growing. We have designated one formation that may be touched in order to satisfy your curiosity. This formation is dead, but we allowed it to become that way in order to preserve the formations throughout the rest of the cave. Once you touch this formation, remember that we cannot touch anything beyond this point. Get it all out of your systems here!

Drapery Column
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Pure calcite is white or translucent. The red-orange tint to most of our formations comes from the iron that mixes in with the calcium bicarbonate solution before it reaches the cave.

A new type of formation-drapery. Drapery forms along an inclined plane. As the water slides down the slanted ceiling or wall, it leaves a streak of calcite behind it. This piece of cave drapery has grown large enough that the calcite has cemented it with the stalagmite growing beneath it, making it a drapery column.

Roughly 100 million years ago, this part of Texas was covered by a shallow sea. Aquatic life forms would sink to the bottom of the water once they died. Their shells were made up of CaCO3 (calcite). As layer upon layer of shells and sediment built up on the sea floor, the pressure of the water and sediment above compacted them to form limestone. The ceiling here is made up of fossilized sea coral, and we can see callianassa shrimp burrow fossils near the center. These are trace fossils-indirect evidence that denotes animal behavior rather than the animals themselves.

Breakdown Room
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This room is made almost entirely of dolomite, which is incredibly absorbent. Formations rarely grow on dolomite unless there is a conduit that allows carbonic acid through to deposit calcite. The most notable feature of this room is along the left-hand side as you enter.The huge crack in the ceiling is a fissure from the Balcones Fault. This fissure served as a major conduit for carbonic acid when the fault line was active about 25 million years ago. (The fault zone was active most recently in January 2013 near Dallas. This occurrence was an exception. The Balcones Fault is designated an aseismic zone, which means earthquakes in this area are very unlikely.) The carbonic acid started dissolving away the limestone, creating what we know as the void of the cave. This process is still going on today.

Your guide may want to do total darkness in this room. It is very important that everyone stands still and quiet when the lights go out. With the removal of one of your senses it is easy to become disoriented, and your guide needs to be able to hear if there is a problem. Some guides prefer to do total darkness in the last room on the tour.

Ice Cream Parlour (First Formation Room)
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The large stalagmite in the center of the mud pit gives this room its name. Up and beyond the ice cream cone is our Kissing Column. These two formations are half an inch apart and, since it takes roughly 100 years for one cubic inch of calcite to grow, we estimate that they will form a column in about 50 years.

Discovery Room
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The Highway Department team drilled into the ceiling of this room in 1963. While the first core sample was being taken, the drill dropped 25 feet down to the floor of this room and got stuck in the flowstone beneath it. Continued drilling indicated that this was not an isolated pocket in the bedrock-there was a large cavern beneath their feet.

They expanded one of the 6-inch diameter holes to a diameter of 24-inches and they lowered Mr. Jack Bigham down on the kelly of the auger rig. He was the first human being to enter Inner Space Cavern. The “Discovery Hole” as we call it was drilled right above a petrified pile of bat guano! Anyone coming and going had to trek through the pile for the first three months of exploration, after which the hole was sealed so construction of I-35 could progress. It was decided that the 33.5 feet of rock above Discovery Room met Highway Department standards, so they did not change their plans for construction. Interstate Highway 35 runs right above this room.

The Flowing Stone of Time is one of the older flowstones on the commercial pathway. The University of Texas took a core sample from the formation in order to date it and determined that this formation is at least 27,000 years old. The funny-shaped rim pool at the bottom is 3 feet deep and can hold up to 125 gallons of water. The water in the cave is filtered by the rock that it trickles through so is 98% pure. It takes roughly 3 days for water from the surface to reach the cave.

Scalloped Ceiling
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As we discussed earlier, carbonic acid is what formed the void of our cave. In some cases, the water present here was flowing as well. Present on the ceiling in this room are cave scallops, which are formed as flowing water erodes away the rock.

We can tell from the large size of the scallops that the water was moving through this section of the cave rather slowly. (The slower the water, the lesser the energy on its surface, which leads to larger wavelengths and larger scallops.) We believe the reason we see scallops here and not in other rooms is because the adjacent sink hole allowed water into the cave at a quicker rate than could seep through into the cave at other locations.

Bone Sink
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There were five sinkholes open to the surface in recent history, the most recent of which is Bone Sink #2. This sinkhole used to be a large room of the cavern, but as carbonic acid ate away at the limestone there was no longer a sufficient amount of rock to support the ceiling, and it collapsed. About 14,000 years ago, wind and water eroded enough dirt into the hole to seal it at the surface.

Between these two events, fauna from the last glacial maximum used this sinkhole to access the cave. The humidity of the cave encouraged vegetative growth around the sinkhole, which would attract animals to graze there. Most probably fell 50 feet from the rim to the cave floor and did not survive, but some paleontologists speculate that saber-toothed cats may have used the cave as a den to raise its young. The bones of at least 44 different species of animal have been found in the cave so far, 11 of which are extinct.

Castle Balcony
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Some guides like to use the formations to tell a fairy tale here. There are flood lines from when the cave flooded in 1997. The water in the cave drains down into the Edwards Aquifer, but when the aquifer fills up there is nowhere else for the water to go. That is when the cave floods. It takes a large amount of rain for the aquifer beneath us and then the cave itself to fill with water. Most recently, the cave flooded in 2007 up to the underside of the balcony.

Painted Wall
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This wall is a retaining wall for the Bone Sink we walked by earlier. Paleontologists were excavating this area when they broke through the debris cone to the rooms on the other side. The wall keeps the loose dirt from the sink hole from sliding down into the pathway. A professor from the University of Texas painted these animals in the style of the ancient drawings of Lascaux Cave in France. Dr. Frary finished the artwork on the wall in July 1966.

All of the animals represented here are animals whose fossils we found in the cave. Starting on the far left we have:

Giant Ground Sloth - Relative of the three toed sloth currently living in the rainforests of South and Central America. They walked on their knuckles and grew to the size of a suburban.

Glyptodont sp. – Relative of the armadillo. They were herbivores and grew to the size of a VW bug.

Equus sp. – Similar to the horse, but smaller. Modern horses are not native to North America, but were brought over by the Spanish.

Black-Tailed Prairie Dog – This is the only species on the wall that is not extinct. Fossil evidence shows that they have remained virtually unchanged for 25,000 years.

Saber-toothed Cat – One tooth is all we found of this animal. Hair does not fossilize, so we do not know if they had stripes or not; therefore, we do not call them tigers.

Peccary – Relative of the Javelina. We found their bones most frequently in the cave.

Camelops sp. – Slightly smaller than the modern camel. Cartilage rarely fossilizes, so we do not know how many humps they had. Camel’s humps are used for storing water. Their environment was much lusher here than where they live today, so they may not have had a hump at all.

Columbian Mammoth – Larger relative of the Wooly Mammoth. Stood up to 14 feet at the shoulder and weighed from 8-10 tons. The largest tusk found to date is 16 feet long. These animals were herbivores.

There is a display case located next to the mammoth containing some of the fossils we found in the cave. The majority of the bones went to Southern Methodist University and the University of Texas for study.

Inner Cathedral
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This is the largest room in the cave. Down over the railing is one of the University of Texas’ five excavation sites that were active from 1965-1970. Some of the fossils are still quite soft, which makes it very difficult to remove them without doing damage. Some of the bones were left where they were found in order to preserve them.

The ladder was used for maintenance and the removal of fossils while the cave was being commercialized. The hole was originally drilled to improve the air quality of the back rooms. There is a little red shed sitting on top of this hole with “Inner Space” painted on it located on the east side of the highway. It is a good landmark if you want to see just how far we travelled underground.

Imagination Station (Lunar Landscape)
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Like with the Castle Balcony, this is a good place to get the kids’ imaginations going. Each guide has different shapes and characters they like to point out to each group, or they may have the class do the finding for them! (Fun side note: There is a word for using the imagination to find shapes in objects that do not inherently have them-pareidolia!)

The other side of the debris cone we passed earlier can be seen at the far end of this room. This sinkhole is approximately the size of a football field and we have walked to the opposite side of it here.

Lake of the Moon
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This is the deepest spot on the commercial pathway at 69 feet beneath the surface. The cavern may get up to 80-90 feet deep. When the cave floods, this room can flood up to the ceiling. The longest soda straw on the pathway is in this room. (A rough estimate of its age, if it is 20 inches long, is 2000 years old.)

If your guide has a chance to show you total darkness here, it helps to have your students sit on the brick wall surrounding the room. Be sure that you are not sitting on a stalagmite! Again, please help us with keeping your students calm and quiet.

This is the last room on the tour. We will walk all the way back to the entrance, stopping occasionally so the guide can turn off the lights and make sure the group stays together. The walk back is a great time to ask questions!