into mark is coordinator of reef watch at noah, the national oceanographic d atmospheric administration, and an expert on the ecology of coral. >> coral is interesting, because they are animal, vegetable and mineral. so you have an animal, and microscopic algae are living inside the tissue. when temperatures get too high, the corals will expel the algae, spit it out into the water column and goes from having a nice algae and tissues to spitting them into the water, a few remaining. coral is lighter. algae gives the coral its colour. most turn white when they kick it out. they have ripped the guts out. and they are starving. this is a stressful event. . >> as we descend below the waves and explore the reef. there are none of the tell-tale whites that signify distress. and still the live coral. instead, interspersed between gardens of healthy coral, we see the skeletal remains. trained of all colour, many covered with lair of algae. others collapse and crumble to the sea floor. he was showing us places where the coral bleached and it's gone beyond bleaching into basically being covered in