Pluto's Chasm was the main horizontal channel in the caverns as water dissolved and eroded layer upon layer of limestone during the formation of these huge chambers over 400,000 years ago. Different solution levels left by the seeping water remain visible along the far wall. Acidic reactions from these accumulations of water will dissolve up to one-tenth of an inch of limestone per year in some caves. This chasm is over 500 feet long, and is 70 to 90 feet in depth.