06/04/2026
15,000 years ago, New York City's backyard
was underwater.
Not ocean water. Not a flood.
A 300-foot deep glacial lake —
larger than the entire modern city
of New York — sitting right next door.
It was called Glacial Lake Passaic.
During the last Ice Age, the massive
Wisconsin Glacier advanced across
northern New Jersey, pushing billions
of tons of earth and rock ahead of it.
This debris plugged the natural drainage
gaps in the Watchung Mountains —
and the Passaic River had nowhere to go.
So it backed up.
For 5,000 years, glacial meltwater
poured into the basin between the
Watchung Mountains and the New Jersey
Highlands — filling it to a depth of
300 feet. The lake stretched 30 miles
from Wayne in the north to Liberty Corner
in the south. 300 square miles of cold,
dark glacial water — right where
suburban New Jersey now sits.
The ridges of Third Watchung Mountain
formed a chain of islands running
down the middle of the lake.
Mastodons and giant beavers roamed
its shores. The Lenape people —
"the original people" — hunted
and fished here as early as
12,000 years ago, when the lake
was still slowly draining.
Then the glacier melted further north.
Around 13,000 years ago it exposed
a gap in First Watchung Mountain
at a place called Great Notch —
near present-day Paterson.
What happened next was catastrophic.
Billions of cubic feet of water
rushed through the gap at once —
carving a gorge through solid
200-million-year-old basalt rock.
That gorge became the
Great Falls of the Passaic River
in Paterson — today a
National Historical Park,
still visible, still thundering.
The lake drained in what scientists
describe as a catastrophic flood.
And then it was gone.
Today the entire lakebed sits beneath
one of the most densely populated
regions on Earth — the suburbs of
the New York City metro area.
Every resident of Wayne, Parsippany,
Morristown, Madison, Chatham,
and Basking Ridge lives on the
floor of a 300-foot deep Ice Age lake.
The Great Swamp National Wildlife Refuge —
just 25 miles from Times Square —
is the last surviving remnant
of that ancient lake bottom.
Next time you drive the I-287
through northern New Jersey —
you're driving on a lakebed.
The world you think you know is wrong.
