Lego gold mine - bubble top indicator?

Just the thing to turn your kids into little gold bugs - Lego's The Mine (my emphasis):

"Dig for gold deep inside the busy mine!

The Mine is a busy place, full of miners digging for gold! Break up the rock with the big drilling machine and transport it out of the mine with the train. Then load it onto the conveyer belt with the crane and haul it away with the truck! There's even a safe to keep the gold inside. Help the miners find even more gold by blasting with the dynamite! Includes 4 minifigures: crew chief, crane driver, driller and truck driver.

Accessories includes 3 dynamite packs, (1 with a timer), 2 detonators, 2 rocks each containing 6 gold nuggets, 36 small rocks in a selection of colors, 2 gold bars, explosion warning sign, storage box, jackhammer, fence with hazard lights, wheelbarrow, broom, shovel, pickaxe, walkie-talkie and coffee cup."