Rupes Altaï is the largest escaprment on the moon. A large cliff that starts in the North near the crater Catharina, and froms a jagged arc towards Piccolomini in the South. It is believed to be part of the rim of Mare Nectaris. When the Sun is high above the lunar surface it is not very conspicuous but when the sun is lower it either shows as a bright  line during first quarter or as a black line during  gibous waning moon.