The Prometheus Pit was made by Professor Prometheus as a portal to the Microverse. It had originally come from the Micronauts and now has appeared in the ongoing series "Minimum Carnage."
Carnage (Kasady) destroys the pit after using it to enter the Microverse. But, Dr. Ketola (the only human survivor of the attack besides Katy Kiernan) is still able to fix the pit so that Venom (Flash) and Scarlet Spider (Kaine) are able to pursue the monster.
The current location of the pit is Houston, Scarlet Spider's city.