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...
- The Martian civilization has clearly developed... under the surface of the planet.
- Their science and technology must be absolutely mind-boggling.
- Quite a lot about Mars, I expect, Nathalie.
[...]