Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.
1. was an underground remnant of Earth's earliest days, sporadically erupting
2. had been an underground remnant of Earth's earliest days and sporadically erupted
3. was an underground remnant of Earth's earliest days, which sporadically erupted
4. would be an underground remnant of Earth's earliest days that sporadically erupted
5. was an underground remnant of Earth's earliest days, having sporadically erupted