(bright music) (waves murmuring) (engine humming) South of 31st Street Beach, a dive boat takes us to one of the lakefronts most surprising secrets.

Going to a shipwreck.

A shipwreck that's just 600 feet from Lakeshore Drive in the Kenwood neighborhood.

And you don't have to be a scuba diver to see it.

It's just inches below the surface on an ancient rocky reef called Morgan Shoal.

(water splashes) (soft music) As you're swimming up to it, it just kind of appears out of the gloom.

- [Philip] Yeah, it is kinda like a sea monster coming out to get you a little bit.

- [Geoffrey] This is the boiler of the doomed steamship Silver Spray.

It's been here since July 15th, 1914.

On that fateful day, Silver Spray was headed to the University of Chicago to pick up business students and take them on a field trip to a Gary, Indiana steel mill.

- So the steamer set out from Chicago, headed down here, clear skies, perfect weather, and ran smack into Morgan Shoal.

Right out here actually.

- [Geoffrey] Fortunately for the students, they hadn't boarded yet.

It was just the crew run aground on Morgan Shoal.

At first, they were more interested in eating lunch than abandoning ship.

But after three days, the wind picked up and the crew jumped overboard as waves smashed the wooden hull against the rock.

- [Philip] Bits and pieces of the Silver Spray then washed up on shore.

Because it was so close, hundreds of people were watching this, and they started picking up pieces of this driftwood and starting fires.

Not just fires but bonfires.

- [Geoffrey] The Chicago Examiner wrote, "Like a Roman holiday, bonfires of the sturdy timbers were built and in their flaring light a feast of the remnants was made."

- Parties broke out, it started to get outta hand.

And finally the Chicago Police had to come through and clean the place out before it got completely outta control.

(both chuckling) - [Geoffrey] The crew swam ashore and all survived.

All that remains is the ship's boiler and propeller.

Totally blew my mind to see it.

And all the old-fashioned like rivets around it.

(gentle music) I was curious about the rock that sank the ship, because most of Lake Michigan's bottom is nothing more than sand and muddy sediment.

Turns out this rock has a story too.

It was once a coral reef.

- If we go back in time, 425 million years- - Oh, just a few years.

- Few years.

Chicago was actually south of the equator and it was in a shallow, tropical sea, underwater.

- We would've had a good tourist trade 400 million years ago.

- It would've been totally different- - It would've been just visit tropical Chicago.

- It would've been just like the Caribbean.

- [Geoffrey] But continental Drift crashed the beach party and moved tropical Chicago to the 42nd parallel.

Over millions of years, the reef died and transformed into a kind of rock akin to limestone called dolomite.

Today this massive dolomite shelf stretches all the way to Buffalo, New York where Niagara Falls spills over it.

Morgan Shoal is one of the rare spots in our area where the dolomite pops up above the lake floor or topsoil.

It's mostly buried hundreds of feet below ground where it serves an important modern day purpose.

It's quarried for gravel and concrete and it supports our skyline.

- [Philip] This bedrock extends throughout the region and some of the skyscrapers in downtown Chicago have some of their pilings driven down into this bedrock.

- If I put my hand right on the rocks down there- - That rock is 425 million years old.

Predates the dinosaurs by hundreds of millions of years.

Glaciers came and went.

It's Lake Michigan now.

I mean, it's seen so much history.

- [Geoffrey] And I could put my hand right on it.

Touch Morgan Shoal and you touch the foundation of Chicago.