Complex sentence: The Icelandic government also believes that there are many more untapped geothermal sources throughout the country, estimating that over 20 Twh per year of unharnessed geothermal energy is available.
Split into simpler sentences: The Icelandic government also believes that there are many more untapped geothermal sources throughout the country. It is estimated that over 20 Twh of unharnessed geothermal energy is available.