This instructable is extremely simple, but exceedingly fun. Every time I tell people about this they look at me like I'm stupid, but after they try it one time they are hooked. All you are doing is dragging someone underwater by a rope behind a team of runners.

I'll do my best to describe why this is so much fun in the theory section, but here is a short list of reasons why you should try this:
-the forces you feel are similar to if you were being towed behind a commercial jet...minus the effects of the freezing cold and thin air.
-even children can effortlessly cover distances underwater they would have never thought possible
-anyone can easily go faster underwater than an Olympic swimmer could go on top of the water
-check out the video!

P7150065.AVI
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Step 1: Theory

Most people have heard of a wind tunnel before, but few people have heard of a water tunnel. The purpose of either a wind or a water tunnel is to test various aerodynamic and geometric parameters on a small scale model before the time and expense it put into developing a full scale prototype. Wind tunnels are difficult to use when testing a model that is much smaller than the intended prototype or when the speed your trying to simulate is much faster than your modeling conditions can generate. It is under these conditions that water tunnels can be used instead of wind tunnels.

By using the pictured set of equations through a process known as Similitude, you can equate all the forces a model would experience during a test to the ones experienced by the real deal. So in this case, we can reasonably say that the forces you experience going a modest 7 mph under water are the same as if you were flying hundreds of miles per hour over the empire state building, just like Superman!
I originally intended to do the math and come up with an estimate of an exact speed, but it quickly became far to complicated to explain in reasonable detail here. A quick comparison of Reynolds numbers (the second equation down) said that 7mph underwater would have to be equal to faster than 2000 mph in air; this isn't the whole picture though and can't be right as 2000mph is hypersonic, haha.

Step 2: Materials

The only materials you will need for this 'ible are:
-rope (at least the length of your pool or the distance you would like to go underwater):
a static or rigid rope (as opposed to a climbing rope) and a heavy bar make it easier and safer on the runners; in the event that someone lets go of the bar it minimizes the potential of the rope releasing all of it's tension and whipping the bar at someone out of the water.
-a sturdy bar about a foot long which you can get a solid grip on (I used a dumbell bar)
-a pool
-a couple friends (the more the merrier)
-goggles are also a plus as they make the experience more interesting

Step 3: Set-up

Tie one end of the rope to the middle of the bar.
Lay the bar at the far end of the pool and run the rope the length of the pool.
Tie some loops/knots on the other end of the rope, just as it leaves the edge of the pool for something to grip on. One for each member of your pulling team (in my case two).
Now pull the bar by the knots at the opposite end until the bar is about 5 feet or so from the end of the pool. Mark this distance at the knot end of the rope CLEARLY, so your runners will know when to stop and not just pull you into the side of the pool; don't worry it doesn't take very long to come to a complete stop in the water.
Now take the bar end of the rope back to the far side of the pool and get ready!

Step 4: Execution

Grab firm hold of the bar, tighten your goggles, take a deep breath, and sink to the bottom of the pool.
Make sure your pulling team has the instructions to let you settle on the bottom for a second and then take off as fast as they can.
As long as you have clearly marked your stopping line they can just run to that without even looking back.
Now let out a cry of disbelief at how much fun that was!

You can do the same thing without sinking to the bottom first but you will just get towed along the surface and it's not nearly as cool.
Kids love this; i have literally dragged 6 year olds the entire length of a 25 yard swimming pool underwater, but be careful...they won't stop at just one ride.