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Last week I received an email letter from an Indiana reader who visits Hawaii often. While here recently, she took a picture of the Hawaiian Hilton Village Lagoon. She writes: "One of my friends asked if the lagoon was swimmable. My reply was definitely not, but I did see a lot of life there. What is living in that lagoon, and why isn't it safe for swimming?"

These good questions turned out to be harder to answer than I expected. After calling and emailing every person, agency and business I could imagine who might have some answers, I still didn't know much. Clearly, this needed some personal investigation.

Before I started my informal study, however, I learned some interesting history about this picturesque pond located between the Hawaiian Hilton Village and the Ala Wai Boat Harbor.

First of all, its name is the Kahanamoku Lagoon. This man-made body of water was part of Henry J. Kaiser's Hawaiian Village development of the early 1950s. Together, he and the state expanded the old Fort DeRussy channel, creating a beach and a 14-foot-deep lagoon to front his new hotel.

When the project was complete in 1956, city officials named the new recreational area the Kahanamoku Beach and Lagoon after Hawaii's Olympic swimming champion, Duke Kahanamoku.

Since I couldn't learn much more from home, I went to the lagoon. It was an enlightening experience.

While walking and wading around the pond's perimeter, I saw about 10 stripebelly pufferfish, an adult wrasse, a baby lizardfish, the distinct tracks of a black-crowned night heron and schools of small, pale tilapia swimming in the shallows.

As to why people don't often swim in the lagoon, I had two theories. One is that the lagoon's abundant upside-down jellyfish, a species that lives standing on its head, bell down, mouth up, often stings people.

These jellyfish host tiny plants in their tissues that supply carbohydrates, but the jellyfish also eat meat. Upside-down jellyfish catch animal meals with frilly arms facing upward in the water. Each of these arms bears stinging cells and several mouths. When a tiny drifting creature comes in contact with an arm, cells sting it, then a string of mucous carries it to the jellyfish's nearest mouth.

When disturbed, upside-down jellyfish release masses of free-floating stinging cells into the water.

A few years ago, a friend and I both suffered stings to our necks seconds after entering the lagoon. I don't know what stung us, but it hurt and I never swam there again.

But in the interest of science, I donned mask and snorkel and again took the plunge. The water was murky in the middle, but I saw lots of upside-down jellyfish near the edges. I did not, however, get stung, even when hand-rescuing an individual stranded near the shoreline. Thus, even though an old, faded sign on the Ewa side of the lagoon warns of jellyfish stings, they aren't a given.

My second theory as to why people don't swim in the lagoon is that people dislike swimming where the bottom is black and mucky, like it is in places there. This dark, soft stuff is the result of a natural decomposition process that occurs when dead organic material breaks down in the absence of oxygen. It doesn't hurt you, but it looks dirty and feels creepy.

So there it is: You can swim in the Kahanamoku Lagoon, but the bottom is squishy and there's a chance of getting stung.

My preference is to watch the lagoon animals from the edge and save swimming for the ocean.