How do we know?
↻ One of the pages with a moving picture
Stalagmites grow in thin layers, like tree rings.
- Rain soaks through soil and becomes a weak acid.
- Over a very long time, it dissolves limestone along cracks, making hollow passages.
- Dripping water leaves rock behind: stalactites hang down, stalagmites grow up.
- When the two meet, they make a column from floor to ceiling.
Stalagmites grow in thin layers, some once a year, like tree rings. Scientists date the layers to learn about Earth’s weather long ago.
Mammoth Cave in Kentucky has over 400 miles of passages: the longest known cave system in the world! Two kinds of rare eyeless fish swim in its underground rivers.
Not all caves are made by rain. Carlsbad Caverns in New Mexico was carved by sulfuric acid, made when gassy underground water met rain.
Wow number
400+ miles of passages: Mammoth Cave in Kentucky is the longest known cave system in the world!
Myth or fact?
Touching cave rocks does not hurt them. Myth. Skin oils stain them and can stop growth.
Stalactite comes from Greek for “oozing out in drops.” Fact. Drip by drip!
Think about it
Caves are dark all the time. How might cave animals find food?
Where in the world?
Mammoth Cave, Kentucky, USA the longest known cave system in the world. Carlsbad Caverns in New Mexico was carved by sulfuric acid — 37.187° N, 86.1° W.
Where these facts come from
Every number on page 26, and who measured it
This page explains how rainwater slowly dissolves limestone into caves.
Solution Caves — The U.S. National Park Service
nps.gov/subjects/caves/solution-caves.htm
We checked it on 31 October 2026
This page explains stalactites and stalagmites, and why we shouldn't touch them.
Stalactites, Stalagmites, and Cave Formations — The U.S. National Park Service
nps.gov/maca/learn/nature/stalactites-stalagmites-and-cave-formations.htm
We checked it on 31 October 2026
This page tells us where the word "stalactite" comes from.
Speleothems — The U.S. National Park Service
nps.gov/subjects/caves/speleothems.htm
We checked it on 31 October 2026
This page tells us Mammoth Cave is the longest known cave system.
Mammoth Cave National Park (U.S. National Park Service) — The U.S. National Park Service
nps.gov/maca/index.htm
We checked it on 31 October 2026
This page tells us about Mammoth Cave's eyeless fish.
Fish — The U.S. National Park Service
nps.gov/maca/learn/nature/fish.htm
We checked it on 31 October 2026
This page tells us Carlsbad Caverns was carved in a special way.
Cave / Karst Systems — The U.S. National Park Service
nps.gov/cave/learn/nature/cave.htm
We checked it on 31 October 2026
This page tells us stalagmites can hold clues about weather long ago.
Speleothem — NOAA's climate-data scientists
ncei.noaa.gov/products/paleoclimatology/speleothem
We checked it on 31 October 2026
This page tells us why cave visitors shouldn't touch the rocks.
Rules for the Cave - What and Why? — The U.S. National Park Service
nps.gov/cave/blogs/rules-for-the-cave-what-and-why.htm
We checked it on 31 October 2026
This page tells us how acid can dissolve limestone.
How do you recognize limestone and marble? [USGS] — The U.S. Geological Survey, the government's Earth scientists
pubs.usgs.gov/gip/acidrain/4.html
We checked it on 31 October 2026
Has this changed?
Caves keep growing, but drip by drip. This answer is not going to change.
Scientists keep exploring caves, so the length of the longest ones may change.
Words from this page
- limestone
- A soft rock, often made of old shell bits, that acid can dissolve.
- stalactite
- A rock icicle that hangs down from a cave ceiling.
- stalagmite
- A rock spike that grows up from a cave floor.
Try it!
Watch acid eat a seashell, the way it eats cave rock.
1. With a grown-up, put a small seashell in a clear cup. 2. Cover it with vinegar. 3. Watch for bubbles. Check again tomorrow.
Why: limestone is often made of old shell bits. Acid slowly eats both, like rain in a cave.
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