This was a hard question. I asked is what could the circular feature be in the satellite image below?
Well, it is a volcanic crater, formed when hot molten rock (called 'lava') is forced though the earth's crust and spills in all directions to create a circular mountain. Earthquakes and explosions of ash and debris are also common with volcanos.The crater is left after the lava has cooled and slowly the vegetation returns.
This crater is called the Diamond Head Crater and this is a picture of it from where we staying in Honolulu:
