Two impressive depressions caused by lava cave collapses are the Puka Street Grotto and the nearby Hochstetter Pond, also known as Grotto Street Pond, in Onehunga. For most of the 200,000 years that the field has been erupting, the planet has been in glacial periods (ice ages) where sea levels were much lower due to water being locked up as ice, and the Waitemata and Manukau Harbours were dry land. All the volcanoes probably erupted on land except for Rangitoto, which erupted during the current interglacial (warmer) period. Tectonic relationships The Auckland region lies within the Australian Plate, about west of its plate boundary with the Pacific Plate. The volcanoes are located south of a geological region called the Northland Allochthon, and with the northern volcanoes located over early Miocene sedimentary deposits of the Waitematā Group of rocks and the southern volcanoes over post Miocene sediments. A large proportion of the volcanoes in the field, particularly those with cone structures, lie within of inferred or known faults, with the qualification that these are inactive historic faults and unlike in many other volcanic fields it is rare for volcanoes to be actually on the fault line. The structure of these Auckland regional faults and the resulting fault blocks is complex but like the volcanic field their locations can be postulated to be related to gravitational variations and where the Stokes Magnetic Anomaly passes through this section of the North Island. The field is part of the Auckland Volcanic Province which comprises four volcanic fields with intra-plate basaltic volcanism starting in the south, at Okete, near Raglan in late Pliocene times (2.7-1.8 Ma). Activity has since moved north through the Ngatutura, South Auckland and Auckland fields since then. Human context Mythology Tāmaki Māori myths describe the creation of the volcanic field as a creation of Mataaho (the guardian of the earth's secrets) and his brother Rūaumoko (the god of earthquakes and volcanoes), made as punishment against a tribe of patupaiarehe, supernatural beings living in the Waitākere Ranges, who used deadly magic from the earth to defeat a war party of patupaiarehe from the Hunua Ranges. In some traditions, the fire goddess Mahuika creates the volcanic field as a way to warm Mataaho, after his wife leaves and takes his clothing. Because of their close association to Mataaho, the volcanic features can be collectively referred to as Nga Maunga a Mataaho ("The Mountains