To dive. It means to descend steeply or plunge, often into water though it may indicate diving through air, or even going down in general, or figuratively going deeply into information or subject matter.

The  simple past tense form of  "to dive" may be "dove" (newer often US and Canadian usage) or "dived" (older and often more British usage), and past participle is usually "dived", but sometimes "dove".

Why isn't it it diven or dave?

Seems like this one was a regular and weak verb that got influenced by similar sounding verbs that had a strong conjugation like "to weave" having the past form "wove" or "to drive" having the form "drove". It was regular and then it wasn't, at least in North America.

That swimmer dove into the pool at last night's swim meet.

The jet dove towards the ground before leveling off.

The hustler took a dive in the pool game in that dive bar.

Foreign stock market numbers dived according to a British news agency.

The cyberpunks dove the system and brought up the missing key.

We took turns diving into the deep end of the pool for pennies.

The cartoon duck dove into the bin of money.

The mermaid dove after the drowning prince.