Underground tank lock

Disclosed are apparatus and methodology for securing access to an underground device such as a storage tank. A lockable blocking plate is secured to a filler cap for an underground storage tank in a manner to inhibit access to the filler cap. The locking mechanism is configured such that the lock and blocking plate may freely rotate with respect to the filler cap. The blocking plate may include a side skirt extending to cover portions of the filler cap and, optionally, portions of a standpipe and coupling for the filler cap. Free rotation is enabled by way of an annular groove in a portion of the locking mechanism, and into which groove one or more locking portions may be inserted, or by way of a freely moving swivel boss attached to the filler cap.