Today we take for granted that the mail will be delivered daily at our door. But many years ago it might have been placed in a tree trunk and 【M1】 ______ underneath a rock. In the early days of the mail no one could be sure about where or when it will arrive 【M2】 ______ At the southern tip of Africa there was once a post office under a rock. At the old days the route 【M3】 ______ from England to India was around the Cape of Good Hope. The journey was stormy and danger. It took six long months. Sailors often wished to 【M4】 ______ send mail home, but they seldom met ships bound back to England. So at the cape the sailors would go on ashore. They headed for a certain large 【M5】 ______ stone. On the stone scratched the words 'Look 【M6】 ______ hereunder for letters.' They would leave their letters there, knowing that the next homeward ship 【M7】 ______ would stop and pick it up. There was another post office like this at the southern tip of South America. During the golden 【M8】 ______ rush days, boats sailed around Cape Horn to California. At Cape Horn was a key(小桶)nailed to a post. Boats coming from the east coast would send for some sailors to this post office. They picked up 【M9】 ______ any letters in the key. At the same time they mailed letters home that boats sail east could pick 【M10】 ______ up. 【M1】