The 'Snowball Earth' hypothesis - the idea that there was a period (sometime earlier than 650 million years ago) during which Earth was completely covered in ice - was developed to explain how (amongst other findings) glacial sedimentary rocks have been discovered in tropical regions.

Although similar ideas had been put forward previously, the specific term 'Snowball Earth' was coined by geophysicist Joseph Kirschvink in a short paper published in 1992.

There are strong arguments for and against the theory. Put another way, some geologists are still trying to find alternative explanations as to how glacial rocks ended up in tropical regions.

Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record.
Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition."

Further reading Wikipedia