Plant it to grow Plumbing. Standard steel pipes, slightly too small for you to climb into.
Plumbing Seed price in Growtopia: sells for about 1/WL, buyers pay about 1/WL (median of 85 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: TI84, SIMALA, IMUPMALL, LUMSI, EEPSON