These ones have also been witnessed and give a similar kind of illusion, except that they are invariably grey or black. They may also make a faint humming or buzzing sound. Often the `cloud’ will be seen to consist of tiny specks. At night some insects do emit light (bioluminescence) and it is now being considered plausible that some of the glowing oval-shaped masses seen at night might well be insect swarms. Whilst some sources have taken this to an extreme, and it cannot be argued that it explains all UAPs away, it is still a source to be contemplated.