Because we didn't get into the canals. The Martian canals are actually canyons. Some of them are over 100 miles deep.

9 seconds sound clip from the Mars Attacks! (1996) movie soundboard.

You can hear this line at 00:18:38 in the Blu-ray version of the movie.

Quote context

[...]

- Coming out of commercial. In five, four, three, two...

- Welcome back. We're speaking with Professor Donald Kessler.

- He's the chairman of the American Academy of Astronautics.

- Professor... isn't it weird that we sent a space probe to Mars... and we didn't even find anyone?

- Well, not really, Nathalie...

- Because we didn't get into the canals. The Martian canals are actually canyons. Some of them are over 100 miles deep.

- The Martian civilization has clearly developed... under the surface of the planet.

- Their science and technology must be absolutely mind-boggling.

- So, what, in your view, Donald Kessler... chairman, are some of the things that the Martians can teach us, professor?

- Quite a lot about Mars, I expect, Nathalie.

[...]