From Khotan they pushed on to the vicinity of Lake Lob, never to be reached again until a Russian explorer got there in 1871.

They halted there to load asses and camels with provisions, and then, with sinking hearts, they began the terrible thirty days' journey across the Gobi Desert.

Marco gives a vivid description of its terrors, voices which seem to call the traveller by name, the march of phantom cavalcades, which lures them off the road at night, spirits which fill the air with sounds of music, drums and gongs and the clash of arms--all those illusions which human beings have heard and seen and feared in every desert and in every age.
What might this be?
A thousand fantasies Begin to throng into my memory, Of calling shapes, and beckoning shadows dire, And airy tongues that syllable men's names On sands and shores and desert wildernesses.
At last they arrived safely at Tangut in the extreme north-west of China, and, skirting the frontier across the great steppes of Mongolia, they were greeted by the Khan's people, who had been sent forward to meet them at the distance of forty days' journey, and so at last they reached his presence in the May of 1275, having journeyed for three years and a half.
The Great Khan received the Polos kindly, listened attentively to the account which they gave of their mission, commended them for their zeal and fidelity, and received the holy oil and the Pope's gifts with reverence.

He then observed the boy Marco, now a 'young gallant' and personable enough, no doubt, and inquired who he was, and Nicolo made answer, 'Sire, this is your servant, and my son,' to which the Khan replied, 'He is welcome, and much it pleases me,' and enrolled Marco among his own attendants.

It was the beginning of a long and close association, for Kublai Khan soon found that Marco Polo was both discreet and intelligent, and began to employ him on various missions.
Moreover, Marco, for his part, found that the Great Khan was always desirous of learning the manners and customs of the many tribes over whom he ruled.