blue sky; open sky
Literally a ceiling painted or imagined as a blue sky; also used for an open-air ceiling or the sky itself.
The open-air hot spring bath has an open sky above, and it feels wonderful.
skyrocketing; unlimited; the sky's the limit
Idiomatic expression for prices, costs, or values that rise without an upper bound. Often used in financial or economic contexts.
At this rate, medical costs will keep skyrocketing without limit.
The price of cryptocurrency is unlimited; the sky's the limit.