There are a new special type of bubbles introduced at level 116, locked bubbles with a keyhole in it. They are a part of a chain and locks the chain. Above the chain you have even more bubbles to burst! But they are greyed out until you have unlocked the lock bubble. A keyhole bubble can be any colour and you get rid of it by matching the colour. Levels with a keyhole bubble usually have to played many times, since you need some luck to manage matching the colour of the keyhole bubble.

You can find these locked bubbles with keyholes at many levels, as examples, view level 129, level 118 and level 120.

Keyhole bubbles are hard, because skills is often not enough, you need luck as well.

A sad little fact about lock Bubbles is that they can not be unlocked as singles with a rainbow bubble. :( But If the lock is paired up with a bubble in the same color, shooting a Rainbow bubble at the paired bubble will unlock the lock.