[iamgopal]

Can’t we just use some form of ultrasonic wave from one end and detect at other end ?

[wincy]

I’d imagine the amount of energy you’d have to push into the water would be insane to hear it 1500 miles away, wouldn’t it?

[naavis]

Whale noises travel thousands of kilometers underwater. https://en.wikipedia.org/wiki/Whale_vocalization

[totalZero]

There's an inverse relationship between wavelength and range, and the higher the frequency the better the resolution.

https://acousticstoday.org/wavelength

[BurningFrog]

Pour in some traceable mix of chemicals, and see if it turns up somewhere.

[tuatoru]

There're these fish called the Devil's Hole Pupfish. Can't do that.

[BurningFrog]

There is a near infinite number of non toxic chemicals to choose from.

[tuatoru]

Ah, but can you show that they're not toxic to Devil's Hole Pupfish? And the population is tiny and fragile--you're not allowed any test subjects.

[BurningFrog]

There might well be 5 separate regulations forbidding it. That's a different question.

The safest class of substances I can think of are unusual isotopes.

They're chemically identical to the regular elements, which is what matters for life forms, but can be detected by scientific instruments.

[alanbernstein]

What other end?

[MichaelCollins]

You'd deafen the poor troglobites.