The Mariner's Valley.

There might be a trench as impressive on Earth as The Mariner's Valley is on the planet Mars (perhaps the similarly named Mariana Trench in the Pacific?). But we can't really know, because our deepest and widest trenches are under billions of gallons of water.

But we have robots on Mars, and flying around it. Based on Observer imagery The JPL Digital Image Animation Laboratory has reconstructed this giant trench that could fit the complete island of manhattan lengthwise on it's floor as a highly detailed computer animation. Google video does it again, take the tour of The Mariner's Valley.