Although usually only a few centimetres thick, microbial mats create a wide range of internal chemical environments. They consist of layers of microorganisms that can feed on or tolerate the chemicals at their level. In moist conditions mats are held together by slimy substances (polysaccharides) secreted by the microorganisms. Some of the micro-organisms form tangled webs of filaments which make the mat tougher. The best known physical forms are flat mats and stubby pillars called stromatolites, but there are also spherical forms.