Sorrow of the Underground City

In an underground ISO city in Siberia that uses mantel energy, a huge explosion takes place which results in a high level of radioactivity. Causing this has been Megatonra, a monster which has been brought from space by Governor X and which grows by absorbing radioactivity. Megatonra has been put under the Earth by Gel Sadra where it has been eating radioactive waste and has grown considerably.