vast system of interconnected canyons on the planet Mars. The system was discovered during, and named for, the Mariner 9 mission in 1971. The canyons extend in an east-west direction for roughly 4,000 km (2,500 miles) just south of the equator between about 30° and 90° W. Individual canyons are typically 200 km (125 miles) across and have walls 2–5 km (1.2–3.1 miles) high. At the centre of the system, several canyons merge to form a depression 600 km (375 miles) across and 9 km (5.6 miles) deep. Some of the canyon walls appear to be fault scarps that formed as a result of crustal movement along faults radiating from the Tharsis rise, an enormous volcanic bulge to the northwest. Erosion, however, has also played a significant role in canyon formation, as indicated by deep gullies cut in the walls. In places, the canyons contain thick sedimentary sequences that may have been deposited in lakes that formerly occupied the canyons. These lakes later may have drained catastrophically to the east, where there is evidence of large floods.