From the brains behind brains on, this is Moment of Um.

Answering those questions that make you go...

I'm Manico Wilhelm.

Here at Moment of Um, we love fishing for questions, and the one we're answering now is a great catch.

It comes from Mark in Miami, Florida.

When lakes form, how do fish get in them?

The vast majority of cases where fish would end up in new lakes has to be through a water connection.

Hi, my name is Megan McPhee, and I'm an assistant professor in fisheries at the University of Alaska, Fairbanks,

and I study fishes in particular salmon. Most fish can't handle being out of water for very long at all. So it's important to remember that what we see as a lake on the landscape didn't always look that way.

So when the glaciers were receding, there was probably a lot of really high-flowing rivers

that were connecting lakes across the landscape. And so that's how we think

most fish ended up in newly formed lakes that followed the recession of the glaciers. And there are

situations. So if we look at little ponds and lakes in Australia that are embedded in deserts,

when we might show up there, we would see them as being really separated. But every now and then, there's really big rainstorms that cause flash flooding,

and all of a sudden all those little water bodies become connected,

and fish have evolved to disperse during those times.

So when there's high flows, they'll start moving and end up back in lakes

that they might have gone extinct from before.

Music and end up back in lakes that they might have gone extinct from before. And guess what?

The oldest lake in the world is in Siberia.

It's Lake Baikal.

And it's also the deepest lake on earth.

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