You could do something to do with Boyle's Law. Maybe something like
filling plastic bottles at different depths and comparing the pressures
exerted on the containers. One on the surface as a baseline so to speak,
one at 15 ft., one at 30 ft. ...

I might even try this myself to help visualize what's going on in my
lungs and sinus.

Karsts, caves and sinkholes

Here's an article to get the thinking process started:

GINNIE SPRINGS, Florida (30 July 2005) -- When he was 13, native Floridian
Wes Skiles discovered the caves of Ginnie Springs and was mesmerized by the
clear water and captivated by the force and amount of water he witnessed
gushing out of the Earth.

He immediately knew he would dedicate his life to underwater exploration.

Skiles, 47, a cave-diving expert and a certified divers since age 16, is now
a veteran underwater cameraman and director and executive producer for the
environment media specialists Karst Productions.

He mapped the underwater cave system at Ginnie Springs in 1976 for his good
friends and Ginnie land owners' Barbara Wray Suggs and her former husband
Bob (the couple is now divorced). That same year, it opened it as a private
attraction.

Some people are excited about shipwrecks, but his passion has always been
underwater caves, says Skiles.

"All the mysteries in life you could want to explore were right there in the
same place," says Skiles.

When he set out as a young man to map the caves at Ginnie, 26 drowning
deaths had been recorded, and Skiles wanted to assist the owners in making
its caves safe enough for others to enjoy this underwater treasure, says
Skiles.

Skiles, who has served as a consultant to the park since 1983, implemented
the "no lights rule" at the Devil Springs system at Ginnie Springs more than
20 years ago, one of many precautionary measures he initiated to reduce the
amount of cave-diving fatalities, changes that included the inception of
scuba and cave-diving instruction at the facility.

The "no light rule," which allows only certified professional divers to
carry lights into the caves, prevents untrained divers from attempting to
navigate the confusing multiple passageways of Little Devil, Devil's Eye and
Devil's Ear, which are too dark to investigate without proper lighting
equipment, says Skiles.

Skiles' mapping of the cave system directly aided in the effort to install
the welded grate in the back of Ginnie Cavern's Ballroom that denies
entrance into the silty, perilous labyrinth of confounding 5-way
intersections beyond it.

The grate does not restrict water flow however, so divers wishing to gain a
sense of the intense force of the 35 million gallons of water a day that
pump through the opening can press their faces up to the grate.

No deaths at Ginnie Springs have been attributed to lost divers since 1976,
the year the park banned non-certified divers bringing lights down in the
Devil Springs system and added the grate, says Skiles.

Skiles has also studied scientific aspects of the springs and his Karst
Productions team (formerly Karst Environmental Services), has developed flow
management techniques and dye tracing to help track the course of the water
and identify potential threats and hazards to the source of this fragile
ecosystem.

Among his work with Karst Productions, Skiles has led underwater cave-diving
expeditions and served as a producer and/or cameraman for the National
Geographic Explorer specials "Mysteries Underground," "Into the Labyrinth,"
and "Endangered Mermaids," and shot and produced original programming for
PBS entitled a "Waters Journey: The Hidden Rivers of Florida," which
examines Florida's river system above and below ground. Skiles just finished
filming on "The Cave," major motion picture release from Sony, due out
August 26.

As an expert underwater filmmaker and modern-day explorer with Karst, he has
traveled to six continents. Yet, with an office just across the street from
the Ginnie Springs' park entrance, he feels like part of the family at
Ginnie Springs. It remains his "sentimental favorite."

What's a science fair?

Posting out of cultural ignorance

How about the effects of depth & pressure?

Not sure how deep you would have to go and you may not have to dive at
all.  But I saw a demonstration once where they took a styrofoam cup to
depth.  When they surfaced with it, it was only a fraction of its
original size (they compared it to a cup that stayed on the surface).
He could do a series of tests with cups.  A cup taken to 33 ft, a cup
taken to 66 ft, a cup taken to 99 ft and create a display showing the
increasing effects of depth & pressure.  He could fashion a container
that would be designed to flood.  Mark of a section of rope with
markings at 33 ft intervals.  Lower the cups in the container at various
depths and observe the effects.

Eurasian milfoil study.  This would take time so a gas study would be
simpler and easier.  It would be interesting to show clear lake bottom and
the invasion of the milfoil; how it takes root and is spread.  He could show
the grown plant and how it protects its area and limits other plant growth.
There is an abundance of information on this subject on the Internet.

Fish.  Fish are always fun.  Identify species and photograph.

Topography and soil types.  Use lake maps to show how the lake bottom varies
in depth and what the bottom of the lake consists of at different depths and
areas of the lake.  Dive for bottom samples and to verify preexisting charts
or maps.

I like the drink bottle idea.  I think that it would probably work best one
of 2 different ways.

First, you could descend to specific depths and fill the bottles with air.
Then, cap them with a balloon.  At the surface, measure the expansion of the
balloon.  If you do this, make sure that you get good, strong balloons or
they'll pop on the way up.

The second option would be to fit a bottle with a balloon at the surface and
then measure how much the balloon intrudes into the bottle as you descend.
For this option, I would probably mark the outside of the bottle at depth
and then, at the surface, I would fill the bottle to each mark with water
and measure the volume of water (this should get you a reasonably good
measure of the volume of air that was remaining in the bottle).

You might do both scenarios and see how much the data varies (i.e. expansion
versus contraction.

Water sampling at various depths and locations. Your state lab might be
happy to do the analysis.

esg

Great idea'sand we appreciate them all.

I like the styrofoam pressure test but we live in an area that has no
direct drops beyond 35 feet (several cave systems go deeper, but nothing
straight down).

He came up with an idea yesterday that fits the fifth-grade type
project. He was getting fitted for dive fins last month and noticed all
the different types. He wants to test them and see if one type is better
than another.

he'll have a subject (*me) swim a 25m straight course at a set depth
while measuring the number of kicks I make with each set of fins and how
long i take. I'll do the course three times to get an average.

Right now we're planning to test: Mare's Quattros, Mares Plana
Aventuras, Scuba Pro Twin Jet, Dive Rite Fin and for true comparison
bare feet.

I'm simplifying and just doing a no current, straight line scissor kick,
but for his purposes that should be fine.

It's simple science but he's interested and that is all that matters.

Thanks again everyone

He could show what happens during "lake turn-over";  the process where
the lake water changes from the summer to the winter.  He could
display "river rocks" that come from the bottom of any river and show
how they are smoothed over by the water current.  He could show which
fish species live where and so what divers might expect to find in
those waters and if he wanted, he could also include things like fresh
water snails / clams, frogs, snakes, etc.  He could also do something
on the effects of everyone using companies like ChemLawn to put
fertilizer and pesticides on their lawn and then what happens when
those chemicals wash into the near-by lake or river.  If he does that,
he can get a list from the state game commission that tells you how
many of various fish you can eat in your area without getting poisoned
from mercury or PCB's.

I'm not real big on the fin thing.  I think the Boyles Law is the best idea.
Do it any way you want, but be sure to take plenty of photos, and make sure
you apply the findings to diving -- why you have to clear your ears when you
dive and why you have to keep breathing, and why an emergency ascent works
as the air expands in your lungs, mask squeeze, etc.

Any updates?