Eye in the belly
A potential eye, such as this one at the marked point, may have a big effect on the almost filling of groups. It doesn't need to be a real eye to make a difference.
So what if White leaves this situation alone? Then Black can play at any time, starting to make a real eye in what used to be White's eyespace. Next, another move at a completes the eye. If White plays there, Black plays b and now the eyespace is almost filled with a bulky five.
Anyhow, the starting position is settled: White is dead.
In this case, with one more space inside, Black can kill by making the eye but White can make a seki.
A famous problem regarding an eye in the belly can be found in the treasure chest enigma
Note on polite English: 'belly' might be considered a little colloquial, or childish, compared with 'stomach', in spoken English. There is a further example (in the centre) at the end of killable eye shapes called 'eye in the stomach'.
However, 'belly' has some appropriate connotations. The belly is traditionally one of the most vulnerable parts of a dragon, and as such is a good place to strike. Also, something is said to 'go belly up' when it dies. Also, belly sometimes means "middle" as in the expression "right in the belly."