eruptions somewhat smaller than mount st. helens ' occurred about 650 years ago at the mono craters , several miles north of the caldera , and about 550 years ago at the inyo craters , which span the caldera 's north rim. ; scientists believe two or three chambers of molten rock , each perhaps 1 1/2 miles wide , are now located beneath the caldera and the inyo-mono craters , at depths ranging from 4 miles to 6 miles . ;