Christmas Island is well known for its biological diversity. There are many rare species of animals and plants on the island, making nature-walking a popular activity. Along with the diversity of species, many different types of caves exist, such as plateau caves, coastal caves, raised coastal caves and alcoves, sea caves, fissure caves, collapse caves, and basalt caves; most of these are near the sea and have been formed by the action of water. Altogether, there are approximately 30 caves on the island, with Lost Lake Cave, Daniel Roux Cave, and Full Frontal Cave being the most well-known. The many freshwater springs include Hosnies Spring Ramsar, which also has a mangrove stand.