Yes, you can visit the Jewel complex at Singapore Changi Airport during a layover, but there is a major catch: Jewel is located in the landside (public) area, not the airside (transit) area. This means that to see the famous "HSBC Rain Vortex" waterfall up close, you must officially clear immigration and customs to enter Singapore. To do this, you should have a layover of at least 5 hours to account for the time spent passing through passport control twice and the 30-minute security screening at your next departure gate. Most Western passport holders can enter Singapore visa-free, but you must complete the "SG Arrival Card" (SGAC) digital form before landing. If your layover is short (under 3 hours), you can still get a glimpse of the waterfall for a few seconds by taking the free Skytrain that runs between Terminals 2 and 3, which passes directly through the Jewel complex. However, for a proper visit to the shops and Canopy Park, you'll need to "enter" the country and then re-enter the transit zone for your flight.