What natural evidence do Mars and its moons provide that support the theories about Martian canals?

Observations from the Astronef reveal that Mars is an aging planet with diminishing seas and expanding continents^[1]. The famous canals, which have long puzzled terrestrial observers, are seen to be semi-regular divisions across the land masses^[1].

These canals are interpreted as the remnants of gulfs and straits that have been widened, deepened, and lengthened by Martian labor^[1]. This suggests their purpose was partly for navigation and partly to keep the inhabitants of the interior within reach of the sea^[1]. The planet also has scarcely any mountains, only low hills, indicating a worn-down surface consistent with such extensive engineering^[1].