National Park Scouts

Tides, Tide Pools & the Edge of the Ocean

Host: Jenni Park: Acadia National Park, Maine Episode 3

Intro

You're standing on a narrow path in the ocean. A small island is behind you. The shore is in front. You've got maybe a quarter mile to go. But the water - the ocean - that was a couple feet away from you five minutes ago. It is getting closer. Cold Atlantic water is pushing in from both sides now, moving fast, covering the gravel faster than you thought it would. And someone behind you says: we should run.

Here's the thing about Acadia National Park's coastline. It does not stay still. Twice every single day, the ocean pulls back and reveals an entire road connecting Bar Island to the mainland. And twice every single day, it swallows that road right back up. Twelve feet of water, up and down. All day, every day.

Today we're gonna find out how. And we're gonna meet some of the most extreme, weird, and wildly creative creatures on the planet - the ones who figured out how to live there. Let's go!

Welcome to National Park Scouts, the show where curious kids discover America's wildest places. I'm Jenni. Today we are back at Acadia National Park on the coast of Maine. In our first episode, we met the peregrine falcon, the fastest animal on earth, nesting right on Acadia's cliffs. Then in episode two, we talked about Acadia's ancient geology - how volcanic magma cooled into pink granite, and then a glacier bulldozed the landscape. Today, we are going all the way down to the very edge, to the place where the ocean meets the land.

We're talking a road that disappears underwater. A neighborhood full of creatures with superpowers. And a hole in a rock that punches back. This is Acadia's coastline. And nothing here stands still.

Bar Island and the Tides

The road the ocean swallows

Just off the shore of Bar Harbor, Maine - close enough you can see it from the sidewalk - there's an island called Bar Island. And here is the thing about Bar Island. Twice every single day, the ocean pulls back and reveals a gravel road connecting it to the mainland. Half a mile of ground that was completely underwater. Just sitting there like the ocean forgot it.

This happens every single day. It has been happening every single day for millions of years. So what causes it?

The moon. The moon is not just sitting there looking pretty - it has gravity, a pulling force, and it is strong enough to pull on the ocean. It actually moves the water. The moon creates two giant bulges in the ocean on opposite sides of the earth. And as the earth spins, different parts of the coast rotate in and out of those bulges. When your part of the coast is there, the water piles up. That's high tide. When you rotate out, the water pulls back. That's low tide. And it happens twice a day, every day, because the earth spins all the way around in 24 hours.

At Acadia, the tidal swing - the difference between high tide and low tide - is up to 12 feet. That's taller than a basketball hoop. And that 12 feet of water goes up and comes back down twice every day.

The Bar Island Road is only crossable for about three hours, centered around low tide. Every single year, people lose track of time exploring the island and the water comes back. They end up stranded, waiting it out or calling for help. The ocean does not care that you were having fun. The ocean has a schedule.

The Wabanaki people - the indigenous people who have lived here for thousands of years - knew these tides long before there was ever a national park. They called this area the clam gathering place. They read the ocean like a clock.

Tide Pool Creatures

Meet the neighbors

That 12-foot tidal swing doesn't just swallow a road twice a day. It creates something incredible. Every time the ocean pulls back, it leaves a whole world behind.

Think about it. These creatures live somewhere that is completely underwater for part of the day, then completely exposed to sun, wind, and air for the rest. The water gets hotter, the salt gets more concentrated, waves crash in without warning. And yet things live here on purpose - because they evolved to be really, really good at it.

Periwinkles

First: periwinkles. They're small snails, about the size of a marble, and they have a built-in door. Scientists call it an operculum - that's a fun word. When the tide goes out and the air gets too dry or too hot, the periwinkle pulls into its shell and seals its operculum door shut. Like locking yourself inside your house and waiting for better weather. It just sits there sealed up, waiting. And when the tide comes back, it opens the door and gets back to business.

Barnacles

Here's a fact that will change how you look at every dock, pier, and boat forever. Barnacles are not rocks. They're not some kind of crusty sea moss. Barnacles are crustaceans - they are related to crabs and lobsters.

When barnacles are tiny larvae, they swim around freely in the ocean. And then they find a good rock and they glue their heads to it permanently. That's it. They cement their actual heads down and they never leave. And then they spend the rest of their lives upside down, poking their feathery legs out to kick food into their mouths. Barnacles eat with their feet, upside down, cemented to a rock for their entire life. I promise I'm not making this up.

Hermit crabs

Hermit crabs are born without shells of their own. So they find an empty snail shell and move in. As they grow, they have to upgrade. They find a bigger shell and move into that one. Sometimes a whole group of hermit crabs will line up by size and each one moves into the next shell up - like a hermit crab housing swap, a whole little neighborhood moving into new apartments at the same time.

Sea stars

Okay, I need to prepare you. This is gonna sound like something from a sci-fi movie, but it is a hundred percent real. When a sea star finds a mussel it wants to eat, it wraps its arms around the mussel's shell and pulls slowly - for hours if it needs to. Sea stars are extraordinarily patient. And when the mussel's shell opens, even the tiniest crack, the sea star pushes its own stomach out through its mouth, through that crack, and into the mussel. It digests the mussel from the inside, then pulls its stomach back in. Scientists call this extra-oral digestion. Extra means outside; oral means mouth. Its stomach goes outside of its mouth to eat.

Let that sink in for a second.

Periwinkles with a door. Barnacles eating with their feet. Hermit crabs who move when they outgrow their house. Sea stars who send their stomachs out for takeout. Every single creature in a tide pool has a superpower - because they had to.

Park Sound

Guess That Park Sound

Okay scouts, it's time for Guess That Park Sound. Listen in. Take your best guess - we'll come back to it after the break.

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That sound was a harbor seal pup - a baby. Right now, in Frenchman Bay just off the coast of Acadia, harbor seal pups are doing exactly what that sounded like: making absolutely sure their mothers can find them on a crowded, noisy rock. Those pups can swim at birth. Within just two or three days of being born, they can already dive underwater for two full minutes. Adult harbor seals can dive all the way down to 1,400 feet and stay underwater for nearly 30 minutes without coming up for air. That's almost five Statues of Liberty stacked on top of each other, straight down on one breath. Harbor seal pups are very cute - you should ask your adult to help you look at pictures. Thank you to the Vancouver Aquarium Marine Mammal Rescue Society. Check out their website at vammr.org and adopt a seal or a cute otter today.

Thunder Hole

The hole in the rock that punches back

Not impressive looking. Not dramatic. Easy to walk right past. And then - when a wave enters Thunder Hole at just the right size and speed - it traps a pocket of air at the back of the cave. The wave keeps coming and the air has nowhere to go. The pressure builds, and then it explodes.

A sound like a thunderclap. You can feel it in your chest. And a blast of water shoots 40 feet straight up into the air. Forty feet - that's a four-story building, from a hole in the rock.

The physics of why it works is the same reason a bottle makes a pop when you uncork it: compressed air forced through a small opening all at once. But instead of a little pop, it's a boom you can hear from half a mile away.

The best time to see Thunder Hole is about one to two hours before high tide - when the waves are big enough to trap air, but not so big they just flood the whole cave and skip the explosion. Timing matters here, just like the tides. People have been coming to Thunder Hole for over 150 years to feel it in their chest, and every single time it works.

That is the edge of the ocean at Acadia. The periwinkle seals its door, the hermit crab finds a new house, the sea star pushes its stomach out - and the ocean, when it finally hits solid rock with nowhere to go, roars. Nothing here is quiet, nothing here stays still, and everything here has figured out exactly how to live in it.

Wildcard Game

Guess That Number

I'm going to give you a measurement, and you tell me - is the real answer higher or lower?

Question 1

The tides at Acadia swing up and down twice a day. Do they swing higher or lower than six feet - about the height of a tall adult?

Higher. The tidal swing at Acadia is 10 to 12 feet - almost double six feet. That's taller than a basketball hoop, going up and coming back down twice a day, every day.

Question 2

Thunder Hole can shoot water into the air. Does it shoot higher or lower than a two-story house - about 20 feet?

Higher. Twice as high. The spray at Thunder Hole can reach over 40 feet - that's a four-story building, straight up, from a hole in a rock.

Question 3

A harbor seal can dive deep when it hits the water. Does it dive more or less than 300 feet?

Way more. Harbor seals can dive down to 1,400 feet - that's nearly five Statues of Liberty stacked on top of each other, straight down. The ocean in Acadia is amazing.

Scout Question

Scout Question of the Day

This week's question: What tide pool creature is your favorite, and why? Let's hear what some of you had to say.

Ella, age 8

My favorite tide pool creature is starfish because a star is my favorite shape. Starfishes are very interesting and each are unique in their own ways.

Emma, age 7

My favorite tide pool creature is a starfish because they are colorful and they grow eyes at the end of their arms.

From Jenni

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What Did You Learn?

What did you learn today that most adults don't know?

The ocean creatures who live in the tidal zone are some of the most adaptable animals on Earth. Their living conditions change completely twice a day, every day. One hour they're underwater; the next they're baking in the sun with waves crashing over them. The salt gets stronger, the water gets warmer - and they don't just survive it, they're built exactly right for it. That's not just cool - it's one of the most remarkable things happening on this planet, in a pool of water the size of your bathtub.

Scout Mission

Your Mission

Every scout needs a mission. Here's yours.

If you're at home, check out the tide chart for Bar Harbor, Maine. Find the next low tide. Calculate your three-hour window on the Bar Island Land Bridge. How long would you have before the ocean takes the road back?

If you're at the park, walk the Wonderland Trail or Ship Harbor Trail at low tide - the Ranger Station can tell you the timing. Find one barnacle, one periwinkle, and one sea star. Don't move anything, don't touch them. Just watch for five minutes and see what happens.

And then it does it all again 12 hours later, right on schedule, whether anyone is watching or not.

Outro

Next time, we're leaving the water behind and heading back in time. There was an event that changed Acadia forever, and the forest is still telling the story of it today. Scouts, your next adventure is waiting.