The guide told us many interesting facts about the glacier and we picked up many more when we visited the very interesting Glaciarium the next day.
The reason why this glacier is famous is because it is reasonably stable - it isn’t receding overall - and since 1917, the glacier builds up the block the two halves of the lake. This causes one side of the lake’s water to rise which puts pressure on the part of the glacier that blocks the lake. Every so often, the water breaks through, creating an ice bridge which then collapses in a really spectacular way. There is no pattern to the build and collapse either so it is really hard to judge when it will happen.
As it had already collapsed in March, there was no way we would see the natural wonder, but just to watch and hear the ice calving into the water (which sounded like a bomb going off) was enough to keep you watching for ages.