Impacts on Earth
To date, there are only 182 confirmed impact structures on Earth (see the Earth Impact Database). In contrast, most other planets consist of thousands to hundreds of thousands of impact structures. Regardless, Earth has undergone billions of years of bolide impacts, mainly taking place in its early history (about 3.8 to 4 billion years ago). Evidence of these early bolide impacts has been erased due to the dynamic nature of the Earth over millions of years. The thick atmosphere, deep oceans, and mobile crust unique to our planet has altered bolides and hidden their structures!
Only a handful of Earth's impact structures are relatively undeformed (for example: Manson, Mjølnir, Lockne, Popigai, and Ries craters), allowing geologists to better study bolide impact processes and their effects on Earth's systems. Most of these craters are exposed on the surface, which are more accessible than submarine impacts, such as the Mjølnir crater. Few submarine impact structures are known, and they are largely understudied due to the deep ocean depths in which they exist.
The relatively young age of the oceanic crust also worsens the chance of submarine impact preservation. Oceanic crust is continually being created at ocean spreading ridges, where molten rock flows out and cools to form new oceanic crust. As the ocean floor spreads apart, older oceanic crust is pushed away and subducted underneath the continental plates, where it is melted and recycled beneath Earth's crust. Thus, any impacts that may have occurred on the ocean floor deep in Earth's history may have been subducted, re-melted, and recycled to create new oceanic crust.
The Alamo Impact Event represents a submarine impact that has since been exposed onto land. Such exposures allow geologists to better study and understand submarine impact processes, making the Alamo Impact an excellent case study. Other submarine impact structures that are now exposed on land include: the Chesapeake Bay (85-km diameter), the Montagnais (45-km diameter), the Mjølnir (40-km diameter), and the Ust Kara (25-km diameter) craters.