Chinese legends mention the mysterious land of Fou-sang, said to lie twenty-thousand Chinese miles (ten-thousand kilometres) in the East, and also fabled to be the house of Paradise and the Tree of Life. The term has variously been interpreted to refer to Siberia, Yesso, Saghalien , Lewchew or Japan or even America by Sinologists; some historians of the eighteenth and nineteenth centuries supposed that Buddhist priests in the fifth century after Christ had explored America's western coast. While historians since the twentieth century have rejected this latter notion of a voyage in the northern sea a millennium before Columbus, Fusou was one of the earlier names given to Japan and still mentioned occasionally to-day; this provides some certain limit of China's knowledge of the east.

Edited. An error of arithmetic that mistakenly implied the distance between China and Fou-sang as equal the circumference of the world is now corrected.

While it's more like 1,800 years ago rather than 900 years ago, there's a particularly bizarre story involving Sun Quan and a Chinese expedition to the semi-mythical islands of Tanzhou and Yizhou. This journey was ostensibly an attempt to rediscover the kingdom founded by Xu Fu around the time of the Qin dynasty (Xu Fu led two expeditions to Kyushu and then just kinda stayed there because he got to be a king) or, to take a slightly more cynical view of it, an attempt to find the Chinese equivalent of Atlantis.

This, of course, was opposed by basically everyone. The most notable figure opposing it was Lu Xun (one of the most important officials in Wu at the time), who stopped just short of calling it folly. Sun Quan, being Sun Quan, did what he did best: he ignored everyone who disagreed with him and ordered an expedition prepared.

So Sun Quan gets two generals (Wei Wen and Zhuge Zhi, the latter of whom had no relation to the other Zhuge fellows floating around at the time), ten thousand men, and a bunch of ships. And then he orders them to sail south.

The order to sail south is kind of important. Yizhou may refer to Taiwan (which the expedition almost certainly reached), but the land variously referred to as Tanzhou or Danzhou was probably Japan. And Japan is obviously not south of Jianye.

The expedition, of course, fails. Zhuge Zhi and Wei Wen limp home, having apparently found Yizhou but not Tanzhou, and Sun Quan (once again being Sun Quan) has them executed.

Some have hypothesized that the second island the expedition was supposed to reach was actually Hainan (probably because Danzhou is a city there), but I'm skeptical of that. In addition to Hainan being known to the Han a century prior to the expedition (Dan'er boasted a military garrison sometime around 100 BC) and it's almost certain that the territory sided with Wu. After all, Wu ruled a significant chunk of Vietnam through the Shi family (which operated out of the Hanoi region) and was actively administering it in 226 when Shi Xie died and Sun Quan ordered the commandery partitioned. So Sun Quan almost certainly knew where Hainan was. I suspect Tanzhou/Danzhou refers to Japan instead; Wu, unlike Wei, didn't have a working relationship with the Japanese.

IANAH but I can offer an English citation that discusses the subject:

http://elib.sfu-kras.ru/bitstream/2311/2438/1/09_Terentjeva.pdf

Note: the article's sources are all Russian although if you can dig them up they should point you to the primary sources from Chinese history. A relevant passage:

In the present article we consider the dynastical stories of the Tang Epoch as a source for researching the Siberian peoples. We should also note that at that time the dynastical chronicles of the Tang Epoch were considered to be an etalon of historical science of China. For the first time the chronicles generalized and systemized the data about vagrant tribes of the Chinese Empire and the peoples living on the territory of modern Siberia.

I'll also note that OP's '900 years ago' happens to fall exactly on the start of the Jurchen Jin dynasty over Northern China in 1115 and the Jurchens were from the Northeast and their power went past what is thought of today as "Manchuria" and certainly their familiarity extended beyond that.

Here is a related post on origins of iron working in the Northwest Coast area. Some information is provided germane to this question.

There's strong evidence for trade between Alaskans and Aleutians and the people of Hokkaido iirc, who themselves had quite a bit of trade with Japan.