The Grotto

At the bottom of the murky lake an underwater cave sits, awaiting discovery. If you tried to enter this cave you would most surely drown. For it would take a good fifteen minutes to swim up the passageway and into the grotto where there is stale air to breathe. No human could hold there breath for that long. Unless you take a drill and dig deep into the earth there is no other way into the grotto, which resembles a large oval cave of slimy rock.
Inner sanctum of the Maris twins. Enter at your own risk.