Moisture-resistant thermoplastic (TW) electrical cable can be used:

Moisture-resistant thermoplastic (TW) electrical cable is designed to withstand exposure to moisture, making it suitable for various installation environments, including underground. While it's true that rigid conduits may provide additional protection and benefits for underground installations, TW cable can still be utilized effectively in this context.

Using TW cable underground is permissible due to its moisture-resistant properties, which help prevent electrical faults that could arise from water exposure. However, the recommendation for rigid conduit stems from the need to enhance durability and protection against physical damage that can occur underground. Rigid conduits shield the cable from environmental hazards and provide structural integrity, making them an ideal choice in many situations.