The most commonly used substrates are common soil, small pebbles, sand, peat, chips of various trees, wood mulch, vegetable fibres (of coconut, for example), or a combination of these. The choice of the substrate depends on the needs of the plants or of the animals, moisture, the risks involved and aesthetic aspects. Sterile vivariums, sometimes used to ensure high levels of hygiene (especially during quarantine periods), generally have very straightforward, easily removable substrates such as paper tissue, wood chips and even newspaper. Typically, a low-nutrient, high-drainage substrate is placed on top of a false bottom or layer of LECA or stones, which retains humidity without saturating the substrate surface.