It's an event that only happens once or twice a year at the most. 而且一年最多也只发生1~2次
that for most of the year they would be unable to cross it. 它们一年中大部分时间都无法穿越这里
Each year explorers chart over a hundred miles of new cave passages. 每年 探险家们都会测绘 上百英里新的洞穴通道
as each year rainwater eats away a little more limestone. 因为每年雨水都会侵蚀掉少许石灰岩
Every year almost 2 thousand million tonnes of sediment 每年大约有20亿吨泥沙在此沉淀
The Amazon transports a billion tonnes of sediment a year, 亚马逊河每年会带来10亿吨的泥沙
For five months of the year it's sealed by an ice sheet over a metre thick. 每年中总有5个月 湖面被1米厚的冰层覆盖
Each year nearly two million animals migrate across the Serengeti plains 每年都会有将近200万只角马 横越塞伦盖蒂平原
but every year some of the world's high rivers are crowded by millions of visitors. 而世界上的高山河流每年都将 迎接成千上万的来访者
There is a tropical downpour here almost every day of the year. 这里全年几乎每天都会降下热带暴雨