We believe this trap was actually used by the IRA (the Irish Republican Army, a terrorist group dedicated to getting the British out of Northern Ireland) as a parting gift for British law enforcement. It works best as a modern trap, but with some adjustment of flavour could be used in a fantasy setting.
The PCs are given the location of a safe house belonging to an enemy group – a rival spy organisation, a demented cult, a supervillain’s gang, etc. When they arrive they discover that the safe house has been cleared out and completely scrubbed down. Whether enough forensic traces of their enemies remain is up to you – but also quite possibly academic.
As the PCs enter, you note that the only sound in the place is a steadily dripping tap, landing in an empty metal sink (there is no plug, so the water just drains away). Drip, drip, drip. As the PCs investigate, the dripping continues – either implicitly or explicitly driving home the complete emptiness of the safe house and thereby mocking their efforts. Play it low-key – you don’t want to give away that this piece of flavour is also bait – but make it mean something more than just scenery to the players. Eventually one of the PCs may get irritated enough by the incessant dripping that they go to turn off the tap – at which point contact is made in the detonation switch, and explosives in the walls and scattered through the building go off.
This trap was originally designed to be completely lethal, so while of course it’s up to you we recommend that the damage be extreme – especially around the trigger.
Whether the PCs have a chance to find the explosives is also up to you – again, in the original traps they were well hidden, often in the walls (set up as the safe house was originally established), or roofspace, or under the floor – or all three. Alternatively they may be hidden in plain sight as caches of (apparently inert) bomb materials.