The age of Silverpit and the confirmed Boltysh crater (65.17 ± 0.64 Ma), as well as their latitude, has led to the speculative hypothesis that there may have been several impacts during the KT boundary. Of the five oceans in descending order by area, namely the Pacific, Atlantic, Indian, Antarctic, and Arctic, only the smallest (the Arctic) does not yet have a proposed unconfirmed impact crater. Craters larger than in the Phanerozoic (after 541 Ma) are notable for their size as well as for the possible coeval events associated with them especially the major extinction events. For example, the Ishim impact structure is conjectured to be bounded by the late Ordovician-early Silurian (c. 445 ± 5 Ma), the two Warburton basins have been linked to the Late Devonian extinction (c. 360 Ma), both Bedout and the Wilkes Land crater have been associated with the severe Permian–Triassic extinction event (c. 252 Ma), Manicouagan (c. 215 Ma) was once thought to be connected to the Triassic–Jurassic extinction event (c. 201 Ma) but more recent dating has made it unlikely, while the consensus is the Chicxulub impact caused the one for Cretaceous–Paleogene (c. 66 Ma). However, other extinction theories employ coeval periods of massive volcanism such as the Siberian Traps (Permian-Triassic) and Deccan Traps (Cretaceous-Paleogene). Undiscovered but inferred There is geological evidence for impact events having taken place on Earth on certain specific occasions, which should have formed craters, but for which no impact craters have been found. In some cases this is because of erosion and Earth's crust having been recycled through plate tectonics, in others likely because exploration of the Earth's surface is incomplete, or because no actual crater was formed because the impacting object exploded as a cosmic air burst. Typically the ages are already known and the diameters can be estimated. Mistaken identity Some geological processes can result in circular or near-circular features that may be mistaken for impact craters. Some examples are calderas, maars, sinkholes, glacial cirques, igneous intrusions, ring dikes, salt domes, geologic domes, ventifacts, tuff rings, forest rings, and others. Conversely, an impact crater may originally be thought as one of these geological features, like Meteor Crater (as a maar) or Upheaval Dome (as a salt dome). The presence of shock metamorphism and shatter cones are important criteria in favor of an impact interpretation, though massive landslides (such as the Köfels landslide of 7800 BC which