When a meteorite struck Earth before humans were around to watch, did it still make a "splat?" Although it's too late to witness the many pummelings our planet has already seen, scientists are still finding the humongous holes left here by long ago impacting space rocks.
At last count, there were more than 170 known impact craters on our planet, according to the Earth Impact Database maintained by the University of New Brunswick in Canada. These puncture wounds are littered over every continent, as well as the seafloor.

There would be countless more if it weren't for Earth's constant remodeling. Plates shift, mountains form, volcanoes erupt and erosion washes over the planet's surface, continually hiding the evidence of most craters.
"If there was no erosion or tectonic activity, we would look like the moon," said Lucy Thompson, a geologist at the University of New Brunswick. "The moon is just pockmarked with impact craters."

Puzzling differences
Scientists think the Earth was bombarded more heavily earlier in the solar system's history, when planets were still forming and bushels of debris were flying madly around. Luckily for us, things have quieted down lately and meteorite impacts are few and far between.
One of Earth's most recently-formed holes is Arizona's Barringer Meteor Crater, created around 50,000 years ago. Though this crater, one of the most famous, awes tourists with its roughly three-quarters of a mile (1.2 km) diameter, it is considered quite dinky on the geological scale.
"That's a nice, simple bowl-shaped crater," Thompson said.
Geologists get really excited about complex craters, such as Manicouagan in Quebec, Canada. Scientists estimate this crater is more than a hundred times wider than Barringer, and was made more than 200 million years ago.
"With large impacts, you have complex craters forming, and instead of having a nice bowl shape, you get a central uplift," Thompson told SPACE.com. "It's like if you drop something in water, you get rings forming, but the middle comes back up."
Scientists want to understand how the rock achieves this without actually becoming liquid or shattering into pieces.

Big and bad
A major heavyweight is South Africa's Vredefort crater, which at 186 miles (300 km) wide, is said to be Earth's largest verified impact crater. At more than 2 billion years old, it is also one of the most ancient.
Other contenders are the 155 mile-wide (250 km-wide) Sudbury Basin in Ontario, Canada, and the roughly 110 mile-wide (180 km-wide) Chicxulub crater, half submerged off the coast of the Yucatán Peninsula in Mexico.
The latter can claim fame as the landing spot of the asteroid that purportedly killed the dinosaurs, along with most life on Earth.

If it weren't for erosion and other geological processes that erase evidence of craters, there would likely be hundreds of thousands of impact craters on the Earth, Thompson said. Scientists are still discovering new craters, especially in remote areas and on the seafloor where evidence of them is easily missed.



The Barringer Meteorite Crater (also called "Meteor Crater") in Arizona is almost 1 mile (1.6 kilometers) wide and 570 feet (174 meters) deep. Originally thought to be of volcanic origin, it was later discovered that the crater was formed by a meteorite just 50,000 years ago – not very long by geologic standards.



Glaciers and snow cover in the Newfoundland region of Canada surround Mistasin Lake, which was created between 34 and 42 million years ago. In the middle of the lake is Horseshoe Island, a chunk of Earth that "rebounded" from the impact, solidifying as a peak at the crater’s center.



This Landsat satellite image shows a false-color view of the landscape around Popigai crater in Russia. The crater, which is currently bounded by a river on its north rim, is a daunting 62 miles (100 kilometers) in diameter and was created between 30 and 40 million years ago.



Two separate -- but likely related -- meteors created this double-impact formation at the same time, some 290 million years ago. The larger, (West) Clearwater Lake, features a prominent ring of uplifted islands; East Clearwater Lake has a similar uplift, but it’s hidden by water.



The circular set of hills seen here is not a crater rim, but the uplift ring in the center of a 13.7-mile- (22 kilometer-) wide crater in Northwest Territories, Australia. In fact, the crater’s rim has been eroded away over a span of nearly 142 million years, but traces of the crater’s remains can be seen as dark circular marks.



In this cloud-obscured image you can see the rim of Bosumtwi crater, located in western Africa. Water fills most of the Bosumtwi crater, which is about 6.5 miles (10.4 kilometers) in diameter and 1.3 million years old.



Thought to be the impact that wiped out the dinosaurs, the Chicxulub crater in the Yucatan peninsula was not easily detected because it’s buried under layers of sediment. But by mapping local gravity and magnetic field variations (seen in this image), scientists determined that an asteroid between 6 and 12 miles (10 and 20 kilometers) in diameter slammed into Earth 65 million years ago, causing near-global catastrophe.



Taken from the vantage point of the space shuttle, this image shows a crater in western Australia. The meteoroid that carved out this crater, which is about 3.2 miles (5.1 kilometers) in diameter, fell on Earth some 50 million years ago.



This crater, located in the Saskatchewan region of Canada, has an outer ring of about 8 miles (13 kilometers) in diameter. Reindeer Lake, in the central portion of the crater, is about 3 miles (5 kilometers) wide and 720 feet (220 meters) deep, and contains an underwater uplift feature. The crater was formed between 50 and 150 million years ago.



This radar image shows the concentric rings of Aorounga crater, which is about 200 million years old. A possible second crater can be seen to the right of Aorounga. Scientists aren’t yet sure whether meteorite or comet fragments made these craters, which are buried beneath the Sahara Desert.



One of the most ancient of all known terrestrial meteor craters, the Siljan crater in Sweden is about 368 million years old and spans roughly 32 miles (51 kilometers). Though it has largely been eroded, four lakes (Siljan, Orsasjön, Skattungen and Oresjön) mark its initial boundaries.



At 43 miles (69 kilometers) in diameter, the Manicouagan Lake in the Canadian province of Quebec is one of the largest well-preserved crater structures in the world. Although it has been eroded substantially, the original crater rim is thought to have been 62 miles (100 kilometers) across. The crater is about 212 million years old



Similar to the Barringer crater in the U.S., the Wolf Creek crater (located in north-central Australia), is young and well-preserved. With a rim that tops out at 82 feet (25 meters) from ground level, this crater was carved out only about 300,000 years ago.



Located in the country of Tajikistan, near the Afghanistan border, Kara-Kul crater is less than 10 million years old. Since the meteor that created it landed in the Pamir mountains, Kara-Kul has a particularly high elevation -- almost 4 miles (6.4 kilometers) above sea level.