What are the four strange phenomena of the Daming Lake in Ji'nan?大明湖四怪是什么？

大明湖，位于山东省济南市市中心偏东北处、旧城区北部，是由济南众多泉水汇流而成，湖面58公顷，公园面积103.4公顷，平均水深2米左右，最深处达4.5米，是繁华都市中一处难得的天然湖泊，与趵突泉、千佛山并称为济南三大名胜，也是泉城济南重要的风景名胜、开放窗口和闻名中外的旅游胜地，素有“泉城明珠”的美誉。

Daming Lake, located in the northeast of Ji'nan city of Shandong and the northern part of the old city, is made up of a large number of springs in Ji'nan. The lake is 58 hectares, the park area is 103.4 hectares, the average water depth is about 2 meters and the deepest reaches 4.5 meters. It is a rare natural lake in the bustling city, with Baotu Spring, Qianfo Hill and Ji'nan three. Scenic spots are also important scenic spots, open windows and famous tourist attractions in Ji'nan, known as the "Pearl of spring city".

　　大明湖的四大谜，由来已久。明末的左诗坛巨擘王象春，就曾经在自己所写的《齐音·大明湖》中有过详细的记载——“湖在城中，宇内所无，异在恒雨不涨，久旱不涸；至于蛇不现，蛙不鸣，则又诞异矣。”后该文又被清朝人收录在《历城县志》中，应该是较早对明湖四怪提出的文字记载。这也就是济南的人家喻户晓的大明湖里“四怪”——青蛙不鸣，蛇踪难寻，久旱不落，久雨不涨。
　　蛇不现：所谓的“蛇不现”，是因为大明湖内的水鸟众多，使得蛇类很难在其中生存下去。
　　恒雨不涨：所谓“恒雨不涨”，是因为大明湖的出水口众多，当水涨时候，自然而然的就流了出去。
　　久旱不涸：所谓“久旱不涸”，是因为大明湖的湖底为质地细密的火成岩，致使源源不断流注湖中的泉水不能下泄。
　　蛙不鸣：所谓的大明湖里“蛙不鸣”之谜，时至今日仍无人能找到一个合理的答案。
　　对于大明湖的蛙不鸣蛇不现的奇怪现象，有着一个美丽的传说，但已被证明系后人杜撰。大约是在清朝乾隆年间，乾隆皇帝下江南路过济南，游览了趵突泉、千佛山等名胜后，于中午来到了大明湖畔的历下亭休息。真龙天子降临大明湖，自然也就惊动了大明湖里的各路神仙，于是百蛙齐鸣，众蛇会聚，大明湖上空更是鸟飞虫跑，一众生灵都希望能够一睹天子龙颜。但已经非常疲惫的乾隆皇帝此刻正想休息，偏偏窗外的蛇啊、蛙啊、鸟啊的不知趣的在这里凑热闹，于是龙颜大怒，一拍桌子，颁下一道圣旨，命令：“蛇归洞，蛙不鸣。”众生灵见龙颜大怒，于是立刻慌慌张张的溜掉了。从那以后，大明湖里就再也见不到蛇了，也听不到蛙鸣了。

The four riddle of Daming Lake has been a long history. Wang Xiangchun, the giant of the left poetic altar at the end of the Ming Dynasty, once wrote in his own "Qi Yin Daming Lake" in detail - "the lake is in the city, the space does not, the constant rain does not rise, the long drought does not dry up; as for the snakes, the frogs do not sing, then it is quite different." Later, it was also included in the records of the county by the Qing Dynasty. It should be written earlier in the four monsters of Ming Lake. This is also known as the "four weird" of the Daming Lake in Ji'nan. The frog does not sing, and the snake trail is hard to find.
Snakes do not show up: the so-called "snake is not present" is due to the large number of waterbirds in Daming Lake, which makes it difficult for snakes to survive in them.
Constant rain does not rise: the so-called "constant rain does not rise" because the Daming Lake has a large number of outlets, and when the water rises, it naturally flows out.

Long drought is not dry: the so-called "long drought is not dry", because the lake bottom of the Daming Lake is a fine texture of igneous rock, causing a continuous flow of water in the lake can not be released.
Frogs do not sing: no one can find a reasonable answer to the mystery of "frog not singing" in Daming Lake.
There is a beautiful legend about the strange phenomenon of the frog in Daming Lake, which does not appear to be snakes, but has been proved to be invented by future generations. During the Qian Long years of the Qing Dynasty, Emperor Qian Long traveled to the south of the Yangtze River in Ji'nan and visited Baotu Spring, Qianfo Hill and other places of interest. At noon, he came to the rest of the pavilion at the Bank of Daming Lake. As the true dragon descended to Daming Lake, the gods of the Daming Lake were shocked naturally, so the hundred Rana ringing, the snakes gathered and the birds flew over the Daming Lake. But the Emperor Qianlong, who was already very tired, was trying to rest at the moment. The snake, the frog, and the bird were not so interested in the window. The spirits of all creatures saw anger at the dragon's face, so they slipped away. Since then, there has been no more snakes in Daming Lake, nor can we hear frogs.